text:
This action might not be possible to undo. Are you sure you want to continue?
Study Notes
Owner: File: Last saved: Email Id. Version
Satish Talim C# - Console Applications 7/6/2001 firstname.lastname@example.com ß3
Copyright Satish Talim 2001-2002, Study Notes. All Rights Reserved.
Notice: This documentation is an early release of the final Study Notes, which may change substantially prior to final release, and is information of Satish Talim. This document is provided for informational purposes only and Satish Talim makes no warranties, either express or implied, in this document. Information in this document is subject to change without notice. The entire risk of the use or the results of the use of this document remains with the user. Complying with all applicable international copyright laws is the responsibility of the user. Microsoft, Windows, Visual Basic, and Visual C++ are either registered trademarks or trademarks of Microsoft Corporation in the U.S.A. and/or other countries. Other product and company names mentioned herein may be the trademarks of their respective owners.
Copyright Satish Talim 2001-2002, Study Notes. All Rights Reserved.
Table of Contents
Table of Contents
1. Introduction ............................................................................................................... 7 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 A New Platform? ....................................................................................................... System Requirements................................................................................................ Purpose of these Study Notes ..................................................................................... Who can use these Study Notes? ................................................................................ Updates to this document........................................................................................... Recommended Sites on C# ........................................................................................ My Workshops on C#................................................................................................. Satish Talim?............................................................................................................ Acknowledgements.................................................................................................... 7 7 7 7 7 7 8 8 8
2. C# Program Elements............................................................................................... 10 2.1 Overview of the .NET ................................................................................................10 2.2 C# and Java ............................................................................................................11 2.3 Our first C# Program – Hello, world............................................................................11 2.4 Naming Guidelines ...................................................................................................15 2.4.1 Namespaces ......................................................................................................15 2.4.2 Classes..............................................................................................................15 2.4.3 Methods ............................................................................................................15 2.4.4 Method Arguments .............................................................................................16 2.4.5 Interfaces ..........................................................................................................16 2.4.6 Class members...................................................................................................16 2.5 Automatic memory management ...............................................................................16 2.6 Comments ..............................................................................................................16 2.7 Blocks.....................................................................................................................19 2.8 Separation ..............................................................................................................19 2.9 Whitespace .............................................................................................................19 2.10 Keywords (74) .......................................................................................................19 2.11 Constants – const / readonly ...................................................................................20 2.12 Variables ...............................................................................................................20 2.13 Naming constants and variables ...............................................................................21 2.14 Escape sequences ..................................................................................................21 2.15 Statements and Expressions ....................................................................................22 2.15.1 Empty statement ..............................................................................................22 2.16 Types....................................................................................................................22 2.17 Predefined types ....................................................................................................25 2.18 Operators ..............................................................................................................30 2.18.1 checked and unchecked operators.......................................................................33 2.19 Operator overloading ..............................................................................................34 2.20 Program Control .....................................................................................................35 2.20.1 The if statement ...............................................................................................35 2.20.2 The switch statement ........................................................................................35 2.20.3 The while statement..........................................................................................37 2.20.4 The do statement .............................................................................................37 2.20.5 The for statement .............................................................................................37 2.20.6 The foreach statement .....................................................................................38 2.21 Console I/O ...........................................................................................................38 2.21.1 Console Input...................................................................................................38 2.21.2 Console Output.................................................................................................38 2.22 Array types............................................................................................................38 2.23 Calling methods – ref / out ......................................................................................43
Copyright Satish Talim 2001-2002, Study Notes. All Rights Reserved.
..............25 Namespaces ....................................................................................................................3...................................... Object Oriented Concepts ...................................................................................................................................................................1..................................................68 4..1 3........................3.........................................................74 4.1.......1....................4 Enums ....................................................91 iv Copyright Satish Talim 2001-2002.............90 5............................................6 Destructors ...........1..............1............................63 4...............1 Finding out information about a File ...........................................................................................................NET Base Classes ...........................1.....................................................................1 Method Overloading ....................3 Constructors......................................................................................62 4...................65 4...........................CONSOLE APPLICATIONS 2...................................83 4........8 Accessing Base Class Members ...................................5 Static Constructors.66 4...........3...................1 Base classes ....................................3....80 4..................................3.......................3.......... 90 5..............12 Access modifiers ...................................................................................3 Interface implementations ..................................................88 5....................... 56 3...2 Interfaces ..................................................................................1 Class Declaration ............75 4.85 4....3.......3....................................................................................57 What are Classes? ..........................................................2 Class base specification ........................................C# .....................5 What is an Object? ...............23..................................................................................................7 Summary of Key Concepts ................................5 Properties ..............................................................14 Accessibility Domain ...45 2................................................74 4.............1..................................71 4................................................................90 5...................................................................................................................................2.....................16 Override methods .......81 4........1................................10 The new modifier ............... 61 4........9 The this Reference....91 5.2 3........................1 Abstract classes .......................................1 Class members ............................................................................1........2 Internal class .........44 2....................2......57 What are Messages? .......................................................................1............26 Namespaces and Assemblies.............45 2................... ...........................4 Calling Base Class Constructors ................................................68 4............................3.............4 3................................................13 Restrictions on Using Accessibility Levels........2................59 4.......1..................3...............3...............2 Variable Method Parameters .......................................1............................... Class and Object ..............................11 Casting between Types ........52 3.....................................2 Sealed classes ......................................................................1..........1.................................................64 4.................15 Virtual methods ......................................7 Inheritance ..........................................................................3....48 2........................3.........1..........................................66 4.........................58 What is Inheritance?..............1.........73 4.2 StringBuilder class...................................1..........................................................................3 Inner Classes ...........................23................3...............................66 4..................1.....................1 The WinCV Tool ....................77 4.......78 4.........................................................................................1......................................6 Assignment ...............3..................................76 4.........................79 4...............1..........3 Class body ........2 Listing Files in a Folder ..51 2....91 5.Single..............................3 3......................................... Study Notes.....67 4..........................................84 4..........................................24 Handling Exceptions ..........1 Class modifiers.............75 4.............................1.....................................................................3..................3..............................1....params ..................................71 4............3 Structs..............1.............1...........63 4..............................................................................1..............................................................65 4.....56 The Benefit of Encapsulation.............................................................78 4....................1................. All Rights Reserved...1..2 Signature ............1...3 File and Folder operations .................................................83 4................61 4.......................3...............................27 Summary of Key Concepts......... Using ..................................................................................................
..92 5.........3..3......................................................... v ........................93 5..........................Table of Contents 5.....4 Networking................4 Reading Text Files ...........................................6 Reading Binary Files..............................................94 5..............94 5......................................3.93 5.................................................3......3 Copying and Deleting Files.3.............5 Writing Text Files...4...........................92 5...93 5..........1 HTTP........................................................................................................................................................... All Rights Reserved........................95 Copyright Satish Talim 2001-2002...................................................................................... Study Notes......4..........7 Writing Binary Files........................................2 Generic Request/Response Architecture.................................................
1. However.NET (available for MSDN Universal Subscribers) and an optional free C# IDE – Sharp Develop. Do not get confused by this. There are many people in the industry.5. Therefore.pune-csharp.com 1.microsoft.asp .4 Who can use these Study Notes? These Study Notes supplement my C# lectures and assume a basic knowledge of Java. a new and exciting technology that finally addresses issues we've been grappling with for years (for example.exe) currently ships with the . Please appreciate. because as of today we have the Beta 1 version of .Chapter 1 Introduction 1. Microsoft's . IE 5. which see a great future to the . I had to refer to all available material – books.NET beta. articles on the net. the deployment and versioning problems of large.NET and C# opens a whole New World of possibilities. I felt the need of a single repository where individuals could go to and know everything about a particular topic in C# – this repository became the Study Notes.NET. all the sample codes in these notes will compile and run from the command line. Study Notes. The .6 Recommended Sites on C# Amit Karmakar (based in Sydney.NET SDK (Beta 1).NET SDK can be downloaded from http://msdn. you will need Windows 2000. an optional Visual Studio . Microsoft . 1. that this is not a book on C#.5 Updates to this document The latest version of this document in .com which will cater to all the technical needs of the C# professionals. to focus on the language and runtime environment and not place any restrictions on your particular development environment. As I studied the C# language. Introduction 1.pdf format is available at the URL: http://www.com/net It's an 80MB download. and so on). discussion groups. All Rights Reserved. 1.NET. The C# compiler (csc. The use of Visual Studio. the PDC version and the pre Beta 2 version. complex systems. Australia) and I have launched a site http://www.htm All feedback / suggestions about this document can be sent to medunet@vsnl. I am confident that it would prove useful to all those interested in learning the C# language. 7 .pune-csharp.3 Purpose of these Study Notes To learn the C# language.com/downloads/index. multi-language development environments.NET has been intentionally avoided.icsharpcode. I kept making notes.1 A New Platform? The crash of the dotcoms and the downturn in the prospects of Java notwithstanding. Copyright Satish Talim 2001-2002. 1. We now have . The C# IDE is available from http://www.2 System Requirements To build and run your C# programs.net/OpenSource/SD/Download/default.NET.
Australia. who has always stood by me and helped me in going through these notes and suggest changes. He was the founder Director of Infonox Software Pvt.8 Satish Talim? Is a software professional with over 23 years of software consulting experience.com 1.devx..com/ http://www. Kanbay Software Ltd.asptoday.com/dotnet/resources/ http://www.C# .thecodechannel.com/net/ http://www.com http://www. in New South Wales.. in Pune. USA. Amit is based in Sydney. Australia and is a Web Developer for the Department of Education and Training.c-sharpcorner.csharphelp. . Study Notes.csharp-station. right from my Java days. His clients include Satyam Ltd. Focus Inc. He is also a certified instructor at the Sun Authorized Java Training center in Pune. Global Electronic Commerce Services amongst many others.com http://www.com/Forums/ http://www.brinkster. IBM ACE.com/ http://www.NET Web Services Window / Web Controls and Components 1. who always believes in me and inspires me to go on to greater heights.7 My Workshops on C# Here are some brief details of the various workshops on the C# language • • • • • • • Console Applications ADO. His current area of interest is C#. Sunil Kelkar.NET WinForms WebForms ASP. to my student and friend Amit Karmakar. Ltd..9 Acknowledgements These Study Notes would not have been possible without the contribution. is an independent consultant and works in Java and C#.com/ http://www. (ISPL) based in California.c-point.microsoft. I would like to acknowledge the help. 8 Copyright Satish Talim 2001-2002.aspwire.CONSOLE APPLICATIONS Here are some other sites related to C# http://msdn. He has been working on Java-based software-export projects and corporate training in Java since 1995. All Rights Reserved. support and generous help of many individuals.com/ http://www. 1.com/ http://www. Sunil. KPIT.csharpindex.
Sudhakar Jalli.Chapter 1 Introduction I want to thank Rahul Waghmare. Copyright Satish Talim 2001-2002. C#. Study Notes. Finally to Microsoft Corporation for giving us this C# language. Chris Maunder. Willy Denoyette. Shafeen Sarangi. C# Language Reference. The C# Programming book from Wrox Press was the first book I read on C#. 9 . whose works I have referred. Ben Albahari. VC++. Tom Archer. To all my students and people on the Internet who provided me with information/feedback. author of Inside C# for his valuable tips and help in clarifying many of my doubts in C#. Sunil Gudipati. All Rights Reserved. Here is a partial list. The book gives a very precise and to the point description of every element in the language. Pramod Singh and Saurabh Nandu for their excellent articles on various aspects of C#. I have been highly influenced by this book and it probably reflects in these Study Notes. a big thank-you. Anders Hejlsberg and Scott Wiltamuth for their excellent work. who made me look at and explore C# ! Rahul. His book helped me understand the internal architecture of C#. author of C# Essentials for clarifying my doubt on byte array. Ashish Banerjee. is a graphics artist by profession. Mahesh Chand. works in Pune and likes to experiment with various computer languages – Java. I acknowledge all the authors.
NET. The BCLs functionality is available to all languages that use the CLR. In addition to C#. The . universal type system called the .NET Framework consists of the: • Common Language Runtime (CLR). .NET Enterprise Servers. All of these languages provide access to the Next Generation Windows Services (NGWS) platform (or the new Microsoft .Object. The . All .NET.Drop and Build features of Visual Basic). the (CTS) also stipulates the rules that the CLR follows with regard to applications declaring and using these types.NET . that ensures seamless interoperability between CLS-compliant languages and class libraries.1 Overview of the . all objects implicitly derive from a single base class defined as part of the CTS.NET infrastructure comprises the .NET Framework class libraries sometimes called the Base Class Library (BCL).NET Framework class libraries at their disposal. Visual C++.NET platform). and the scripting languages VBScript and Jscript (infact 21 languages so far). and shares the compiled binary versions of his/her components 10 Copyright Satish Talim 2001-2002.NET shifts the focus in computing from a world in which individual devices and Web sites are simply connected through the Internet to one in which devices.NET base class library is shared between all programming languages.NET Framework class libraries are very vast. it has complete access to the same rich class libraries that are used by seasoned tools such as Visual Basic and Visual C++. C# aims to combine the high productivity of Visual Basic (C# is a Rapid Application Development Language like Visual Basic. separate from other processes on the machine. NGWS software development kit defines a "Common Language Subset" (CLS). . The . and computers work together to provide richer solutions for users.CONSOLE APPLICATIONS 2. Because the same . this means that even though C# is a new language. In the world of . It supports the Drag. • • • C# is a programming language for developing applications for the Microsoft's . the elegance of Java and the raw power of C++.NET development platform. Microsoft Visual Studio. The code that runs within the CLR runs in an encapsulated and managed environment.NET Framework. C# Program Elements C# (pronounced "C sharp") is a simple. In fact.NET that lets us develop these applications is the . The part of . . a developer can take his knowledge of this library with him as he migrates from language to language. The . C# is provided as a part of Microsoft .NET applications function. In addition to defining all types. This base class called System. 2. A developer picks the .NET and C# everything in the CTS is an object. object oriented. modern. Think of the CLR as the virtual machine / runtime environment in which .NET Common Type System (CTS).NET supports Visual Basic.NET. the . and Microsoft Windows. writes components in it. For C# developers. not only is everything an object but. The . Also take note of the fact that a reference can be null) programming language derived from C and C++.NET infrastructure refers to all the technologies that make up the new environment for creating and running robust.NET language that he/she likes most. All Rights Reserved.NET Framework.NET languages have the . and type-safe (a reference (when not null) is always guaranteed to point to an object that is of the type specified and that has already been allocated on the heap. C# itself does not include a class library. Study Notes. even more importantly. scalable. services.NET Framework class libraries include support for everything from file I/O and database I/O to XML and SOAP.C# . distributed applications.
NET class library are very similar to classes and namespaces in the Java class library. A .2). class Hello { public static void Main(string[] args) { Console. Both have garbage collectors.NET. someone else probably will in order to enjoy the reduced development costs associated with platform independence.NET runtime has been released for your platform then all the . This makes the . world"). C# applications cannot run without the . some of the classes and namespaces inside the .NET platform language-neutral – i. rather than to native code which can be understood by the computer's processor.3 Our first C# Program – Hello.e. as mentioned earlier. as in hello.WriteLine("Hello.cs or any name you want. This IL code is then compiled to native code either when the application is installed. } } The default file extension for C# programs is . 11 Copyright Satish Talim 2001-2002.cs. Now if a . Once this standard is in place. The C# compiler has an option to automatically produce XML-formatted code documentation using a special comment syntax. We'll now write the "Hello.NET Platform for Linux is expected soon. Even if Microsoft Corporation doesn't produce the Macintosh. for example.NET platform has been released for Win NT/2000.cs using System. promote the grouping of classes. interfaces and implementation together in one file so that developers can edit the code more easily. Both are in charge of authenticating code before it's executed.NET programs are compiled to an intermediate language (IL). Both languages. or when the application is run.cs. vendors worldwide will be able to use it to develop C# compilers that target their own platforms.NET runtime for other platforms soon. you need to choose an editor – I suggest Sharp Develop (details of which are mentioned in Section 1. modules written in C# would also be compatible with those written in VC++ and VB. In fact. Microsoft has submitted a C# standard for ratification by ECMA (the European Computer Manufacturers' Association). All Rights Reserved.NET languages. As of today. Until the time a separate runtime environment is released for .Chapter 2 C# Program Elements with developers using other . The name of the program can be hello. When executed. As present the . The one edge that Java has over C# is platform independence. Both handle objects in much the same way: via references. 2. it's speculated that Microsoft would release the CLR for platforms other than Windows.NET Software Development Kit (SDK) on every machine you can to run your programs. There are parallels between Java's JVM and C#'s CLR – both are in charge of interpreting bytecode.txt too! (Java programmers should note this).2 C# and Java There's no doubt that C# resembles Java. Study Notes. UNIX. However. . and IBM runtimes. world" example application to get to know the basic syntax and structure of writing C# applications. Additionally. world Before you write your first C# application. the components use the . Some differences though: C# uses operator overloading. Program Hello. you can name your program as Hello.NET programs will run on your platform.NET runtime. A threat to Java? 2. but this situation is expected to change. MS plans to release the . you will have to install the Full . Windows is the only platform for which an IL runtime has been developed so far.NET runtime component for security and memory management. The basic premise is quite simple: all . type-safe enumeration.
Having source code files with any other extension would require you to always change that filter (or update the Registry setting for Visual Studio). the steps for building the programs will always work.cs extension.00. First. Second.cs files such that they appear in the Windows Explorer as "C# Source files" and when you open them. By simply typing hello you will see the output of the program as: Hello. When you define a class in C#.cs which produces an executable program named hello.exe). world Close examination of this program is illuminating: • • C# programs (Note: program code only) are case-sensitive and in free format. This provides two benefits. With regards to the C# compiler allowing the use of files that don't have the . you end up with a fully encapsulated bundle of functionality that you can easily drop into any other development environment without worrying how that language processes include files or if it has a mechanism for including files within files. The C# compiler produces an executable program named hello. using a different extension would require you to manually create the association and in the case of a .NET. When you install Visual Studio. This way when you write a C# class. with no object files.cs extension.exe. this is simply the compiler team allowing you the freedom to use whatever extension you wish.2204. an automatic file association is created for the . 3.9030 [CLR version 1.C# . Console (character only and graphics free) applications are normally used for creating driver programs for components that you are testing and a great way to start learning C#." We would be using the C# command-line compiler (csc. If you haven't made any typos.21] Copyright (C) Microsoft Corp 2000. The extension makes it obvious to anyone browsing a particular folder that the file is C# source code. for instance.txt file.NET environment. Why is this a good thing? It enables the writers of classes to create highly mobile code. Study Notes. learning the different compiler options will help you in the long run in situations where your editor doesn't provide you complete control over this step. you must define all methods inline—header files do not exist. All rights reserved. Such a program can be compiled with the command line directive csc hello.cs extension for three reasons: 1. Once again.exe.exe) throughout these notes. . take an entire class and drop it into an Active Server Pages (ASP) page and it will function just as it would if it were being compiled into a desktop Windows application! • 12 Copyright Satish Talim 2001-2002. Author Inside C#: "Most people would recomend that you use the . it means that no matter what environment you're using.CONSOLE APPLICATIONS An important note by Tom Archer. 2. Using this "one-stop programming" approach. Something like: Microsoft (R) Visual C# Compiler Version 7. C# compiles straight from source code to executable (. which is one of the key concepts of the . display) files with a . It also means that we are not doing anything specific to Windows development. that might not be what you want. The Visual Studio File Open filter for C# files is set to look for (and therefore. will appear in your command prompt window. All Rights Reserved.00. you can. they are automatically opened with Visual Studio.
and WriteLine is a static method (a static method can access any static member within the class. but the whole package.. } The parameter of the Main method is a string array that represents the command-line arguments used to invoke the program. } public static void Main(string[] args) { } } Copyright Satish Talim 2001-2002. C++. A "using" directive enables unqualified use of the members of a namespace. Console is a class defined in that namespace. Every method also has a parameter list following its name with zero or more parameters between parenthesis. This is shown in the example ManyMain. Functions and variables are not supported at the global level. All it does is to tell the compiler where it can look for unresolved class names. This namespace contains the Console class referred to in the Main method.. unlike C.cs below: • • • • • • using System. Every method (Main method here) must have a return type.WriteLine. In Main() the first character is a capital M.. such elements are always contained within type declarations (e.Console. you can expect compiler errors. } (c) It can also take arguments as in public static int Main(string[] args) { . In this case. so that it can be invoked on its own. It is the entry point of your program.. It can either be void or return an int. The "Hello. System is the root of the . class GF { public GF() { Console. world" program uses Console.NET base class namespace. The Main function is a static member of the class Hello. where the program control starts and ends.NET runtime.WriteLine("In GF class"). class and strict declarations). directive references a namespace (similar to a package in Java) called System that is provided by the .. return 0.Chapter 2 • C# Program Elements The using System. this array does not include the name of the executable (exe) file. If you have zero or more than one Main() in a program. it is "void". Every C# application must have a method named Main defined in one of its classes. The latter form allows your program to read command-line arguments. using is very similar in concept to Java's import keyword. without the creation of an instance of the class.g. Namespaces provide a hierarchical means of organizing the elements of a class library. It is declared inside a class or struct. All Rights Reserved. Unlike Java. a package has to physically map to a directory). return 0. The Main method can be written without parameters or with parameters... Study Notes. There are three ways to declare the Main method: (a) It can return void as in public static void Main() { . 13 . in C# you cannot import a single class. Notice that. However. The Main method is the place where you create objects and execute other methods.WriteLine as a shorthand for System. It must be static. The Main function can have any access modifier and can be written as static void Main() too. but it cannot access an instance member) defined in that class. which means that "Main" does not return a value. The Main method is a static member of the class Hello. } (b) It can also return an int as in public static int Main() { . The static qualifier makes the Main() method a class method.. The Main method can be written without parameters or with parameters. the components of a namespace name do not have to map onto directories (remember in Java.. What do these identifiers denote? System is a namespace. It doesn't matter which class contains the method—you can have as many classes as you want in a given application—as long as one class has a method named Main.
All rights reserved. you'd use this switch: csc MultipleMain. The program does not use #include to import program text.2204.00. C# does not itself provide a class library. as declaration order is not significant." as a separator in compound names such as Console.21] Copyright (C) Microsoft Corp 2000.WriteLine("In F class").C# .exe' has more than one entry point defined: 'F. For example.9030 [CLR version 1.cs /main:S Changing the switch to /main:F would then use the F. C# programs use ".cs(16. Why would you want to do that? One reason is to place test code in your classes. You can then use the /main:< className > switch with the C# compiler to specify which class's Main method is to be used. Test. Dependencies between programs are handled symbolically rather than with program text. The /main compiler option specified a class in which to look for a Main method. world" output is produced using a class library.22): error CS0017: Program 'Test. Study Notes.WriteLine("In S class"). • • • Note: Observe the following 14 Copyright Satish Talim 2001-2002.exe' has more than one entry point defined: 'S. The program does not contain forward declarations. All Rights Reserved.cs(7.22): error CS0017: Program 'Test. However.cs(26. . This system eliminates barriers between programs written in different languages. Instead. C# uses a common class library that is also used by other languages such as Visual Basic and Visual C++. the Console class could be written in C# or in some other language. } public static void Main(string[] args) { } } class S : F { public S() { Console. the Main method has to be defined as follows: public static void Main(){ } • The "Hello.Main(string[])' Test.Main(string[])' Test.22): error CS0017: Program 'Test. } } This program when compiled gives the error: Microsoft (R) Visual C# Compiler Version 7.Main method.CONSOLE APPLICATIONS class F : GF { public F() { Console.Main(string[])' The designers of C# included a mechanism by which you can define more than one class with a Main method.WriteLine. } public static void Main(string[] args) { S son = new S().Main method is used as the application's entry point. To compile this application such that the S.00.exe' has more than one entry point defined: 'GF. Forward declarations are never needed in C# programs.
where the first character is capitalized . you'll see the name of the current file being compiled. Hello. Save the above file as hello. name classes by using nouns that describe the class's problem domain.Win32.NET Framework SDK) for a more detailed description. In cases where the class is more generic (that is. 2. Copyright Satish Talim 2001-2002. this description will give you enough to make the error clear.Chapter 2 C# Program Elements a. and employ mixed casing with an initial capital letter—for example. and you sell two products— training and a software development—name your namespaces JavaTech. JavaTech. Many times. therefore. Then for each product. CS0234.on all methods.2 Classes Because objects are supposed to be living.SoftwareDevelopment. after the error code.NET Framework SDK: Microsoft.cs and compile as csc Hello. This strategy might result in more long-winded names. breathing entities that have abilities. but remember that the users of your code need only specify the using directive to save typing.cs and compile as csc rubbish. Next.4 Naming Guidelines Choosing a stable and easily understood naming convention will enable you to write code that's easier to read and. All Rights Reserved.cs or csc hello. less specific to the problem domain) than that—for example. you'll see a short description of the error. a type that represents an SQL string—use Pascal casing .4.cs and compile as csc Hello. but this will at least give you a place to start. Save the above Hello.cs and run the program as rubbish e.3 Methods Use Pascal casing . 2. to maintain. 15 .Training and JavaTech. Methods are meant to act—they carry out work. Study Notes. b and c as above For a to e above we get the same output i. It might end up slightly different from what is presented here. Save the above file as rubbish.cs and run the program as hello or Hello c. Compile the above program as csc hello. world program as Hello. 2. Therefore.cs and run the program as Hello b.4.where the first character is capitalized.1 Namespaces Use your company or product name. Examples of this are PrintInvoice and OpenDatabase. world Compiler Errors: Here is what to expect when the compiler encounters syntax errors in your code. which will prevent name collision with other products. You can also search for the error code in the . Finally. let the names of your methods depict what they do.e. create a nested namespace with its embedded types. An example of this can be found in the . followed by the line number and column position of the error. you'll see the error code as defined by the compiler—for example. Make the class as public and try a. 2. First.cs and run the program as Hello or hello d.NET Framework SDK Documentation (which is installed with the .4. If you're in the business of selling component software. A standard is still evolving. create a top-level namespace that is the same as your company name. If your company is called Javatech.
Note: The Finalize (inherited from Object) method allows an Object to attempt to free resources and perform other cleanup operations before the Object is reclaimed by the Garbage Collector (GC).C# . 2. therefore. Once a variable is assigned null. */ The C-style comment can occur within a line and can span more than one line. 2. The .5 Interfaces Use Pascal casing (where the first character is capitalized) on all interfaces. // Comments can appear on an independent line or as part of a statement too. Manual memory management is both time consuming and difficult. 2. A common technique is naming interfaces with adjectives. It's common to prefix interface names with a capital "I"—for example. that object no longer has any live references to it (i. All Rights Reserved.5 Automatic memory management Manual memory management requires developers to manage the allocation and de-allocation of blocks of memory.4.4. but is not required to do so. it won't be assigned to any variables or stored in any arrays).NET Garbage Collector (GC) is permitted to clean up immediately. You don't have to do any explicit freeing of memory. C# has a garbage collector that looks for unused objects and reclaims the memory that those objects are using. Study Notes. . you just have to make sure that you are not holding onto an object you want to get rid of. Give meaningful names to arguments. once you finish with an object. necessary cleanup operations should be done elsewhere.CONSOLE APPLICATIONS 2.6 Class members Use camel casing. in which the first letter is not capitalized.4 Method Arguments Use Pascal casing (where the first character is capitalized) on all arguments. 16 Copyright Satish Talim 2001-2002. This program displays the string "Hello World!" on the screen.4. Hence. 2. That way if you have a method that takes as an argument something called Foo.6 Comments The first line contains a comment: // A "Hello World!" program in C# Text following a comment // up to the end of the line is ignored by the compiler. you can differentiate it from the internal representation of that variable by creating the internal member named foo.e. it become eligible for garbage collection. In the vast majority of cases. IComparable. You can also comment a block of text by placing it between the characters /* and */. for example: /* A "Hello World!" program in C#. C# provides automatic memory management so that developers are freed from this burdensome task. This method may be ignored by the Common Language Runtime. this automatic memory management increases code quality and enhances developer productivity without negatively impacting either expressiveness or performance. // This line is commented out // is extra.
unless you use the /doc:filename compiler option to tell it to generate the XML. The following sample XMLsample. In source code files. event. </summary> /// <returns> /// Return results are described through the returns tag. have no effect.0 with this feature.</returns> Copyright Satish Talim 2001-2002. They are there to document what your program does in plain English. } } /// <summary> /// Description for SomeMethod. C# Program Elements Comments are not considered when your program is compiled. Study Notes. public SomeClass() { // TODO: Add Constructor Logic here } /// <summary> /// Name property </summary> /// <value> /// A value tag is used to describe the property value</value> public string Name { get { if ( myName == null ) { throw new Exception("Name is null").NET 7. In C# you can document the code you write using XML. All Rights Reserved.</summary> /// <remarks> /// Longer comments can be associated with a type or member /// through the remarks tag</remarks> public class SomeClass { /// <summary> /// Store for the name property</summary> private string myName = null. 17 .</summary> /// <param name="s"> Parameter description for s goes here</param> /// <seealso cref="String"> /// You can use the cref attribute on any tag to reference /// a type or member and the compiler will check that the /// reference exists.cs program provides a basic overview of a type that has been documented. C# is the only programming language in Visual Studio. /// <summary> /// Class level summary description goes here.Chapter 2 The extra // above. member such as a field. property. delegate. // XMLDoc\XMLSample.cs using System. or interface. Block comments can't be nested. lines that begin with /// and that precede a user-defined type such as a class. </seealso> public void SomeMethod(string s) { } /// <summary> /// Some other method. The source code file that contains Main is output first into the XML. or a namespace declaration can be processed as comments and placed in a file. } return myName. or method. These will be treated as normal comments by the compiler. The compiler also supports a set of documentation tags that you can use within XML comments.
0"?> <doc> <assembly> <name>XMLSample</name> </assembly> <members> <member name="T:SomeClass"> <summary> Class level summary documentation goes here.xml.String)"> Notice the use of the cref attribute to reference a specific method </seealso> </member> 18 Copyright Satish Talim 2001-2002. </seealso> </member> <member name="M:SomeClass.myName"> <summary> Store for the name property</summary> </member> <member name="M:SomeClass. . as shown below: <?xml version="1. </summary> <returns> Return results are described through the returns tag.</summary> <param name="s"> Parameter description for s goes here</param> <seealso cref="T:System.SomeMethod(System. /// </summary> /// <param name="args"> A list of command line arguments</param> public static int Main(string[] args) { // TODO: Add code to start application here return 0.</returns> <seealso cref="M:SomeClass. Study Notes.CONSOLE APPLICATIONS /// <seealso cref="SomeMethod(string)"> /// Notice the use of the cref attribute to reference a /// specific method </seealso> public int SomeOtherMethod() { return 0.cs /doc:XMLsample. } } To compile the sample code.C# . } /// <summary> /// The entry point for the application.xml The /doc option allows you to place documentation comments in an XML file. This will create the XML file XMLsample.String)"> <summary> Description for SomeMethod.</summary> <remarks> Longer comments can be associated with a type or member through the remarks tag</remarks> </member> <member name="F:SomeClass.SomeOtherMethod"> <summary> Some other method. All Rights Reserved. type the following command line: csc XMLsample.SomeMethod(System.String"> You can use the cref attribute on any tag to reference a type or member and the compiler will check that the reference exists.
they can be used as part of a longer token. fields and members respectively. and so on. All Rights Reserved.9 Whitespace C# is a freeform language. The abstract case base catch bool char keywords break checked byte class are: Copyright Satish Talim 2001-2002. if a programmer is bent on using one of these words as an identifier of some sort. 2. Study Notes. it is a very bad idea in terms of human readability. C# uses the semicolon to indicate the end of a statement. you can't use them as identifiers for something else. It is a standard C# programming style to identify different blocks with indentation. 2.String[])"> <summary> The entry point for the application. 2. { // C# block of code } Every executable statement in C# will be within one or more blocks.7 Blocks C# code is organised into blocks (or sections). Every time you enter a new block. While their names are not keywords. F and M prefixes denote types.Main(System. When you leave a block.Name"> <summary> Name property </summary> <value> A value tag is used to describe the property value</value> </member> </members> </doc> All the members of the assembly are denoted by <member> tags. You don't have to indent anything to get it to work properly. However. you can use an initial uppercase letter. for example: public int abstract_int. the overuse of these names may make your meaning unclear to future people working on your programs. The T.10 Keywords (74) Because keywords have specific meaning in C#. because C# is case sensitive. While this is possible. you should de-indent by two spaces. class names. Whitespace characters are space. You specify the beginning and end of a block using curly braces.8 Separation As in C. and you can see how the compiler has added the full name of the member as a name attribute. tab or newline. Appropriate use of white space makes your programs easier to read and understand. such as variables. 2. you should indent your source code by preferably two spaces. There are numerous Classes defined in the standard packages. </summary> <param name="args"> A list of command line arguments</param> </member> <member name="P:SomeClass. Also. Blocks define scope (or lifetime and visibility) of program elements. constants. 19 .Chapter 2 C# Program Elements <member name="M:SomeClass.
Symbolic constants are simply words that represent values in a program. which specifies that the member can have its value set once only. A const variable cannot take as its value a variable that is not const.C# . const int x = 100. One problem you may encounter is that values of const members are calculated at compile-time. when you define a field with the readonly keyword. Study Notes. // OK – compiler knows value of x By default. 2. After that point. public class Account { public readonly string sortcode. All Rights Reserved. The continue statement will terminate the current iteration. in a program. const int y = x*5.12 Variables Variables are values that can change as much as needed during the execution of a program. the field cannot be changed by either the class itself or the class's clients.06. you have the ability to set that field's value in one place: the constructor. do.11 Constants – const / readonly A constant is nothing more than a value. The break statement will exit from the immediately enclosing for. so you can't use const to define a member whose value can't be set in this way. foreach or switch statement. The readonly fields that we define are instance fields. while. variables 20 Copyright Satish Talim 2001-2002. C# has provided a readonly modifier. . and afterwards is read-only. const members are static. public Account(string code) { sortcode = code. C# does not support labeled break as in Java. Hence. const tells C# that this data object is going to be a constant. Hence. causing a jump to the end of the block forming the body of the loop. meaning that the user would have to instantiate the class to use the fields. Such values as the numeral 2 and the string constant "C#" are sometimes called hard-coded values or literals. we are setting the sortcode to the string value passed into the constructor. What does a programmer do when the need arises for a field with a value that won't be known until run time and should not be changed once it's been initialized? To get around this. In the example below. One reason you need variables in a program is to hold the results of a calculation.CONSOLE APPLICATIONS const do explicit float implicit is null params ref sizeof this uint using continue double extern for in lock object private return static throw ulong virtual decimal else false foreach int long operator protected sbyte string true unchecked void default enum finally goto interface namespace out public sealed struct try unsafe while delegate event fixed if internal new override readonly short switch typeof ushort The break and continue keywords: These can be used to control iteration. for example: const float SALESTAX = 0. } } 2. that stays the same throughout the program's execution.
For instance. the escape sequence means that the " mark should be printed and it does not indicate the end of the string literal. This is useful to provide a means of communication between objects or for storing data that all objects need to reference.cs below clarifies this point: using System. Console. but also a wide range of other characters for representing most international characters. Following the first character. and false for bool. C# actually have three kinds of variables: instance. and a value.WriteLine("Value is: {0} ". a type.Chapter 2 C# Program Elements are locations in memory in which values can be stored. 2. The scope extends to the end of the current block. No object really owns this variable but any can read or write it. '\0' for characters. You can't access them via an object. Unicode is a character set definition that not only offers characters in the standard ASCII character set. They have a name. a-z. as you can in Java. Variable definitions can go anywhere in a method definition. C# allows simple and compound variable declarations. } } A class name or other identifier can be no longer than 512 characters.14 Escape sequences The \" in a string literal is an example of an escape sequence.13 Naming constants and variables The rules for choosing constant and variable names (identifiers) are that every C# identifier must begin with one of the characters: A-Z. the @ prefix can be used to avoid such a conflict. In C#. as in 'C'. As a special case. No matter how many times a class gets instantiated. the following two identifiers are equivalent: int @int The example SI. 0 for numeric variables. class and local variables. only one copy of this variable will exist. In this example. In general. Instance variables are defined outside the confines of any method declaration. a-z. instance variables are available to all methods within the class and are automatically initialised when an object is created. 2. although typically variable declarations for a given block are placed immediately after the opening curly brace ({). A static variable belongs to the class itself (not any individual object). static members must be accessed through a class name. Your C# program will not compile if you try to use an unassigned local variable. C# does not have global variables. Study Notes. class Tst { static void Main(){ int @int = 20. You can't declare another variable with the same name in the current or any nested blocks. The \ character indicates the beginning of an escape sequence and the character or characters that follow are interpreted in a special way. but the character is not considered part of the identifier that it precedes. All Rights Reserved. 0-9 An identifier must not clash with a keyword. the rest of the identifier can use any of these characters: A-Z. 21 . Local variables are local to some method. @int). Their initial value depends on the type of variable: null for objects of classes. C# language uses the Unicode character set. are not automatically initialised and are lost once a method terminates. Copyright Satish Talim 2001-2002.
Not all expressions are permitted as statements.WriteLine("Test. The value that F returns is simply discarded.. but merely compute a value (which will be discarded).16 Types A C# program is written by building new types (typically classes) and leveraging existing types. Study Notes. .. Statements are free format.C# . Thus. can run over more than one line without any need for continuation characters. In particular. The value computed by the expression. is discarded. when you add two numbers together or test to see whether one value is equal to another. The example using System. A statement forms a single C# operation.CONSOLE APPLICATIONS 2. if (done) goto exit. class Test { static int F() { Console. Expressions are built by combining variables and operators together into statements. All Rights Reserved. } 2. if any.. either those defined in the C# language itself or imported from other libraries.for example. } } shows an expression statement. and these in turn are made up of statements. empty-statement: . which contain methods..1 Empty statement An empty-statement does nothing. An empty statement can be used when writing a while statement with a null body: bool ProcessMessage() {. are not permitted as statements. Each type 22 Copyright Satish Talim 2001-2002. expressions such as x + y and x == 1 that have no side effects. The call to the function F made from Main.15. and are terminated with a semicolon.F"). an empty statement can be used to declare a label just before the closing "}" of a block: void F() { . Execution of an empty statement simply transfers control to the end point of the statement. but in C# they can also be used as the destination for a goto operation. } static void Main() { F().. constitutes an expression statement. } Also. return 0. A statement may have a label associated with it. 2. the end point of an empty statement is reachable if the empty statement is reachable. An empty statement is used when there are no operations to perform in a context where a statement is required. Normally labeled statements are used in switch statements. An expression statement evaluates a given expression. . exit: .15 Statements and Expressions A C# program is structured into classes. C# statements end with a semicolon.} void ProcessMessages() { while (ProcessMessage()). Statements sometimes return values ..
delegate types. uses 8 bytes of // memory on the stack for x and y a = new PointR(). but the new instance of a PointR remains in memory until the garbage collector determines it is no longer referenced Copyright Satish Talim 2001-2002. a. float etc. The object on the heap uses 8 // bytes of memory for x and y. and thus possible for operations on one variable to affect the object referenced by the other variable. Generally. whereas variables of the reference types store references to objects. uses 4 bytes of // memory on the stack to hold address PointV b. Reference types include class types. An example SRMEM. C# supports two major kinds of types (including both predefined types and user-defined types): value types and reference types. are structs (simple type actually alias structs found in the System namespace). which // will be 0 for both x and y. Data members and function members that can be used on the type itself are called static members. With value types. // Local value-type variable. interface types. Value types include simple types such as char. Study Notes. // Assigns the reference to address of new // instance of PointR allocated on the // heap. such as storing the // object's type & synchronization state b = new PointV(). and it is not possible for operations on one to affect the other. Those data members and function members that require a type to be instantiated are called instance members. } class Test { static void Main() { PointR a. } // Value-type declaration struct PointV { public int x. With reference types. and an // additional 8 bytes for core object // requirements. class PointR { public int x. Value types are allocated on the stack and Reference types are allocated on the heap.x = 7.cs clarifies this point: // SRMEM. You can expand the set of simplentypes by defining your own structs and enums.x = 7.cs // Reference-type declaration using System.Chapter 2 C# Program Elements contains a set of data (typically fields) and function members (typically methods). /* } } At the end of the method the local variables a and b go out of scope. the variables each have their own copy of the data. The default constructor // for both PointR and PointV will set // each field to its default value. it is possible for two variables to reference the same object. y. and array types. // Local reference-type variable. you must vreate instances of a type to use that type. 23 . // Calls the value-type's default // constructor. All Rights Reserved. Value types differ from reference types in that variables of the value types directly contain their data. which combine to form the modular units that are the key building blocks of a C# program. b. y. int.
.WriteLine(a. This ability to treat value types as objects bridges the gap between value types and reference types that exists in most languages.. Console. PointV d = b. All types in C# . All types – including value types – can be treated like objects. int values are simply 32 bit values.C# .CONSOLE APPLICATIONS */ // Assignment to a reference type copies an object reference. This is because in the case of unboxing. this functionality is available on-demand.value and reference – inherit from the Object superclass. Each value type has a corresponding hidden reference type. For programs that need int's to behave like objects. An int value can be converted to object and back again to int. an object box is allocated to hold the value.x = 9. This example shows both boxing and unboxing. The process of turning a reference type back into a value type is called unboxing. the cast is necessary so that the compiler can verify that the cast is valid per the specified variable type. // unboxing } } is more interesting. For programs that don't need int values to act like object.Object in the . The example BUB. // boxing int j = (int) o. d. // assignment to a value type copies an object value. C# provides a "unified type system". and does so without introducing unnecessary overhead. an object could be cast to any type. You can assign values of any type to variables 24 Copyright Satish Talim 2001-2002. The object type is based on System. When unboxing—converting from a reference type to a value type—the cast is needed.ToString()).NET Framework. The example using System. When a variable of a value type needs to be converted to a reference type.x). c.x = 9. All Rights Reserved. It is possible to call Object methods on any value. whereby the object class is the ultimate base type for both reference and value types. This type system unification provides value types with the benefits of object-ness.WriteLine(b.. object o = i.cs class BUB { static void Main() { int i = 123. When an object box is cast back to its original value type. which is automatically created when it's cast to a reference type. // Prints 7 } } The process of turning a value type into a reference type is known as boxing. Study Notes. even values of "primitive" types such as int. Therefore. and the value is copied into the box. } } calls the Object-defined ToString method on a constant value of type int. the value is copied out of the box and into the appropriate storage location. PointR c = a.WriteLine(3. // Prints 9 Console. class Test { static void Main() { Console.x). Unboxing is just the opposite.
56D. float value = 1. bool value = true. char value = 'h'. interface. ulong val1 = 12. float double bool char decimal Single-precision floating point type Double-precision floating point type Boolean type.23M. 2.Object class. Decimal value = 1. ulong val4 = 78UL. long val1 = long val2 = 34L. ulong val2 = 34U. The conversions between types may be implicit or explicit. inherit from the System. 12. Ushort val1 = ushort val2 = 34U. Type object string sbyte short int long byte ushort uint ulong Description The ultimate base type of all other types String type. The object data type is the type to and from which objects are boxed. All Rights Reserved. and can define new reference types via class. Thus. a char value is a Unicode character Precise decimal type with 28 significant digits C# is a strongly "Typed" language. and provides examples of each. Double val1 = 1. 12. This is enforced at compile time.Chapter 2 C# Program Elements of type object. most of which will be familiar to C and C++ developers. a bool value is either true or false Character type. and delegate declarations. All data types. There are rules that define what operations are legal in order to maintain the integrity of the data you put in a variable. Study Notes. ulong val3 = 56L. The table below lists each of the predefined types. Developers can define new value types through enum and struct declarations. 12. byte val1 = byte val2 = 34U. uint val1 = uint val2 = 34U. int val = 12. 12. 25 . Sbyte val = 12. String s = "Hello".17 Predefined types C# provides a set of predefined types. Short val = 12. Implicit numeric conversions can be performed automatically and are guaranteed to succeed and not lose information. predefined and user-defined.23F. a string is a sequence of Unicode characters 8-bit signed integral type 16-bit signed integral type 32-bit signed integral type 64-bit signed integral type 8-bit unsigned integral type 16-bit unsigned integral type 32-bit unsigned integral type 64-bit unsigned integral type Examples Object o = new Stack(). The implicit numeric conversions are: Copyright Satish Talim 2001-2002.23 double val2 = 4. all operations on variables are performed with consideration of what the variable's "Type" is.
From float to double.Int32 by the CTS. or decimal. uint. double. ulong. or decimal. All data types. From byte to short. floating point types. double. Conversions from int. ushort. uint. and the types bool. From char to ushort. and will be the same size on any system. From long to float. The signed integral types are sbyte. the unsigned integral types are byte. though it is considered good style to use the keyword rather than the complete system type name.CONSOLE APPLICATIONS • • • • • • • • • • From sbyte to short. float. All Rights Reserved. This is in contrast to Java. //convert to 2 bytes All the types have fixed sizes. There is no advantage to using one technique over the other. or decimal. long. double.NET Framework. This in particular means that values of the other integral types do not automatically convert to the char type. or decimal. or decimal. double. long. double. In the .C# .NET runtime. . and decimal. double. float. and ulong. int. uint. char. and the floating point types are float and double. or decimal. double. inherit from the System. The predefined value types include signed and unsigned integral types.Int32 x = 123. ushort. ulong. long. long. the keyword int is shorthand for a struct named System. except object and string. long. From ushort to int. or long to float and from long to double may cause a loss of precision. most languages choose to implement aliases to those types. float. You can assign values of any type to variables of type object. For example. The predefined reference types are object and string. ulong. From uint to long. int. From short to int. a reference-type instance has an eight-byte overhead. The other implicit numeric conversions never lose any information.NET languages. short. float. // 4bytes short s = (short)x. System. The C# type keywords and their aliases are interchangeable. and need to be explicitly converted to and from object wrapper types. float. There are no implicit conversions to the char type. From int to long. double. int. double. In C#. While the CTS is responsible for defining the types that can be used across . Explicit numeric conversions require a cast and runtime circumstances determine whether the conversion succeeds or information is lost during the conversion. are referred to as simple types. where the basic types have no object-like behaviour at all. From ulong to float.Object class. and long. An example of this: int x = 123456. uint. you can declare an integer variable by using either of the following declarations: int x = 123.Object in the . predefined and user-defined. uint. For example. but will never cause a loss of magnitude. The two names can be used interchangeably. or decimal. The object type is based on System. int. All the types in the table. or decimal. float. which stores the object's type and 26 Copyright Satish Talim 2001-2002. or decimal. float. Study Notes. each of the predefined types is a shorthand (alias) for a system-provided type. ulong.
. this method returns true if the two types are identical and have the same value.cs clarifies this point. } public class MyClass2 { public static void Main() { object a.WriteLine("Same Object if (s1 == s3) // false Console.ToString()).WriteLine("Same Object if (s3 == s4) // true Console.WriteLine("Same Object if (s1.NET Framework. a = new MyClass1 (). int i1 = 1. Content (s1 and s2)"). Example ObjClass. but subclasses can override this method to support value equality instead. Content (s1 and s3)"). Object o1 = new Object(). int i2 = 1. if (o == o1) // false Console. The following sample shows how variables of type object can accept values of any data type and how variables of type object can use methods on System. The example ObjEquals. 27 Copyright Satish Talim 2001-2002.i). MyClass1 ref_MyClass1. Each reference to a reference type instance uses four bytes of storage.WriteLine(ref_MyClass1. // an example of boxing Console. Console. String s1 = "Hello".WriteLine("Same Object Reference (o and o1)"). a = 1.WriteLine(a).WriteLine(a. The default implementation of Equals supports reference equality only.WriteLine("Same Object if (s1 == s2) // true Console. } } Equals() is a very important method in Object class. String s3 = "World".Equals(s3)) // false Console. Content (o and o1)"). Study Notes. Console. Reference (s1 and s2)"). public class MyClass1 { public int i = 10. int i3 = 2. public class ObjEquals { static void Main() { Object o = new Object(). String s4 = s3. String s2 = "Hello".Equals(o1)) // false Console. Console. Reference (s1 and s3)").WriteLine(). using System.Chapter 2 C# Program Elements temporary information such as synchronisation lock state or whether it has been fixed from movement by the garbage collector. All Rights Reserved. Reference (s3 and s4)"). ref_MyClass1 = (MyClass1)a. Console.Object from the .WriteLine(a.Equals(s2)) // true Console.WriteLine("Same Object if (s1.GetType()).cs using System. In the case of value types.WriteLine("Same Object if (o.
the equality operators (== and !=) are overloaded to compare the values of string objects. string b = "hello". The [] operator accesses individual characters of a string: char x = "test"[2]. This makes testing for string equality more intuitive.Equals(i3)) // false Console. which makes it easy to write.WriteLine("Same Object Content (i1 and i2)").WriteLine("Same Object Content (i1 and i3)").CONSOLE APPLICATIONS if (s3.WriteLine("Same Object Content (s3 and s4)").C# . if (i1.Equals(i2)) // true Console. For example: // a string literal The advantage of @-quoting is that escape sequences are not processed. including escape sequences: string a = "\\\u0066\n". // The + operator concatenates strings: string a = "good " + "morning".ToString() One can declare a string as follows: string str = "Hello".txt" // rather than "c:\\Docs\\Source\\a. The string reference type requires a minimum of 20 bytes of memory. . double it: @"""Ahoy!" cried the captain.Equals(s4)) // true Console. // call object.WriteLine( a == b ). Quoted string literals are enclosed in double quotation marks ("): "good morning" // a string literal // backslash. int n = 5.String in the NGWS Framework. letter f. // output: True -.Boolean. Console. string s = n. string is an alias for System. a fully qualified file name: @"c:\Docs\Source\a.ToString(). The C# object class is very similar to Java's Object. The bool keyword is an alias of System. if (i1. Study Notes. for example. not references. Although Boolean values require only 1 bit (0 or 1).WriteLine( (object)a == b ).txt" To include a double quotation mark in an @-quoted string. } } The string type represents a string of Unicode characters. quoted and @-quoted. string a = "hello". If you declare as follows: string str = new string("Hello").same value False -. The bool type is used to represent boolean values: values that are either true or false. new line and can contain any character literal. String literals are of type string and can be written in two forms. Although string is a reference type.different objects Console. // x = 's'. @"good morning" @-quoted string literals start with @ and are enclosed in double quotation marks." // "Ahoy!" cried the captain. they 28 Copyright Satish Talim 2001-2002. All Rights Reserved. you will get a compiler warning.
a real numeric literal on the right-hand side of the assignment operator is treated as double.e. hexadecimal escape sequence. However.Chapter 2 C# Program Elements occupy 1 byte of storage since this is the minimum chunk addressing on most processor architectures can work with.. or vice versa.5F. The char keyword is an alias of System. Common examples include financial calculations such as tax computations and currency conversions. The char type is used to represent Unicode characters. Precision is 15-16 digits. the bool type is distinct and separate from the integral types. char MyChar = (char)88.7F). The decimal keyword is an alias of System. In C#. U can be used to denote unsigned literals. Precision is 7 digits. char MyChar = '\x0058'. and a bool value cannot be used in place of an integral value.Single. The byte type is an Unsigned 8-bit integer. you will get a compilation error because you are attempting to store a double value into a float variable. No standard conversions exist between bool and other types. and L longs.77M. Study Notes. The float keyword denotes a simple type that stores 32-bit floating-point values. the number is treated as a double. and helps eliminate the all-too-common C++ coding error in which a developer mistakenly uses "=" when "==" should have been used. the example int i = . In particular. char MyChar = '\u0058'. for example: double x = 3D. a real numeric literal on the right-hand side of the assignment operator is treated as double. thus generating a compiler error. use the suffix d or D. The sbyte type is an Signed 8-bit integer. By default. You can use F to denote single precision (i. is invalid because the expression i = 0 is of type int. The precise decimal type is denoted by an M suffix. The float keyword is an alias of System. The inclusion of bool makes it easier for developers to write selfdocumenting code. // Character literal // Hexadecimal // Cast from integral type // Unicode The decimal type is appropriate for calculations in which rounding errors are unacceptable. The double keyword is an alias of System.. The byte keyword is an alias of System. The sbyte keyword is an alias of System. if you want an integer number to be treated as double. Constants of the char type can be written as character literals. use the suffix m or M.. to initialize a float variable use the suffix f or F. All of the following statements declare a char variable and initialize it with the character X: char MyChar = 'X'. The double keyword denotes a simple type that stores 64-bit floating-point values. If you don't use the suffix in the previous declaration.SByte.Decimal. and if statements require an expression of type bool. You can also cast the integral character codes. All Rights Reserved. A variable of type char represents a single 16-bit Unicode character. if (i = 0) // Bug: the test should be (i == 0) G(). for example: decimal myMoney = 300. 12. F(i).Double. 29 . Without the suffix m. If you want a numeric real literal to be treated as decimal. for example: float x = 3. or Unicode representation. Therefore. It occupies 16 bytes of memory. By default.5m.Byte. Copyright Satish Talim 2001-2002.Byte System. as in 12.Char. The decimal type provides 28 significant digits.
Int64. Study Notes. uint. When you use the suffix U or u. It's a signed 64-bit integer. ulong. the following expression evaluates as long: 898L + 88 The ulong keyword denotes an integral type. // Calling the method with the int parameter // Calling the method with the long parameter You can use the long type with other numeric integral types in the same expression.Int32. The ushort keyword denotes an integral type. like this: uint myUint = 123U. In 30 Copyright Satish Talim 2001-2002.C# . for example: MyMethod(5). Consider. When you use the suffix L or l. When an integer literal has no suffix. It's an unsigned 16-bit integer. In the case it is long because it less than the range of ulong. The int keyword is an alias of System. it is uint. When an integer literal has no suffix. its type is the first of these types in which its value can be represented: int. You can also use the suffix L or l with the long type like this: long myLong = 4294967296L. 2. in which case the expression is evaluated as long (or bool in the case of relational or Boolean expressions). There are four basic kinds of operators: arithmetic. bitwise. it is uint. C# has 50 built-in operators.UInt64. The long keyword is an alias of System. You can declare and initialize a long variable like this example: long myLong = 4294967296. It's a Signed 32-bit integer. The short keyword is an alias of System.18 Operators Operators are special symbols that are commonly used in expressions. the literal type is determined to be either uint or ulong according to its size. It's an unsigned 32-bit integer. It's a signed 16-bit integer. its type is the first of these types in which its value can be represented: int.UInt32 System.UInt32. The uint keyword denotes an integral type. long. uint. In this example. In the preceding example.Int16. MyMethod(5L). You can also use the suffix u or U. The ulong keyword is an alias of System. You can declare and initialize a variable of the type uint like this example: uint myUint = 4294967290. All Rights Reserved. . for example. The long keyword denotes an integral type. ulong. the type of the literal integer is determined to be either long or ulong according to its size. the following overloaded methods that use long and int parameters: public static void MyMethod(int i) {} public static void MyMethod(long l) {} Using the suffix L or l guarantees that the correct type is called.UInt16. long.CONSOLE APPLICATIONS The int keyword denotes an integral type. For example. The int keyword is an alias of System. A common use of the suffix is with calling overloaded methods. relational and logical. It's an unsigned 64-bit integer. In this example. it is of the type long because it exceeds the range of uint (see Integral Types Table for the storage sizes of integral types). The ushort keyword is an alias of System. The short keyword denotes an integral type.
and expressions. The ternary operator takes three operands and uses infix notation (c? x: y). The binary operators take two operands and all use infix notation (such as x + y). R is Relational. The Associativity for Assignment. L is logical. != and ?: is from Right to Left. C# does add a few operators of its own. exists. Examples of operands include literals. Operator overloading permits user-defined operator implementations to be specified for operations where one or both of the operands are of a userdefined class or struct type. and new. C is Conditional. Study Notes. Only one ternary operator. The order of evaluation of operators in an expression is determined by the precedence and associativity of the operators.Chapter 2 C# Program Elements addition. ==. operators with the same precedence level will be evaluated from left to right in the given expression. Certain operators can be overloaded. A student has suggested a mnemonic: "Ulcer Addicts Really Like C A lot" where U is Unary. All Rights Reserved. The following table summarizes all operators in order of precedence from highest to lowest: Copyright Satish Talim 2001-2002. In general. There are three types of operators: • • • Unary operators. 31 . many operators can be overloaded by the user. local variables. The unary operators take one operand and use either prefix notation (such as –x) or postfix notation (such as x++). Expressions are constructed from operands and operators. ?:. A lot is Assignment. A is Arithmetic. Binary operators. The operators of an expression indicate which operations to apply to the operands. Anyone familiar with the operators of C will have no problem using C#'s. *. -. for all others it's Left to Right. /. Examples of operators include +. fields. Ternary operator. thus changing their meaning when applied to a user-defined type.
. all binary operators are left associative. number = number / 10. An example using % operator – P3. while (true) { sum = sum + number % 10. Study Notes.cs // P3. x = y = z is evaluated as x = (y = z).CONSOLE APPLICATIONS Category Primary Unary Multiplicative Additive Shift Relational Equality Logical AND Logical XOR Logical OR Conditional AND Conditional OR Conditional Assignment Operators (x) + * + << < == & ^ | && || ?: = *= /= %= += -= <<= >>= &= ^= |= / >> > <= >= is x. Precedence and associativity can be controlled using parentheses. the associativity of the operators controls the order in which the operations are performed: • • Except for the assignment operators. meaning that operations are performed from left to right. x + y + z is evaluated as (x + y) + z. int sum = 0.C# . For example. meaning that operations are performed from right to left.cs using System. } Console.y ! % f(x) ~ ++x a[x] --x x++ (T)x x-new typeof sizeof checked unchecked != When an operand occurs between two operators with the same precedence. For example. The assignment operators and the conditional operator (?:) are right associative. but (x + y) * z first adds x and y and then multiplies the result by z.WriteLine("Sum = " + sum). For example. class P3 { public static void Main(String[] args) { int number = 548. All Rights Reserved. if (number == 0) break. } } 32 Copyright Satish Talim 2001-2002. x + y * z first multiplies y by z and then adds the result to x.
33 . because floating-point types are based on IEEE 754 and so have provisions for representing infinity and NaN (Not a Number).cs . Decimal arithmetic overflow always throws an OverflowException.WriteLine("Unchecked output value: {0}". -. Integer division by zero always throws a DivideByZeroException. the most significant bits of the result are discarded and execution continues. } // Returns -2 } public static void Main() { UCS myObject = new UCS(). MyMethodCh()). // OverFlowTest. what happens depends on the execution context. Copyright Satish Talim 2001-2002. myObject. In general: Integer arithmetic overflow either throws an OverflowException or discards the most significant bits of the result (see below). class OverFlowTest { static short x = 32767. which can be checked or unchecked. Floating-point arithmetic overflow or division by zero never throws an exception. Console. Decimal division by zero always throws a DivideByZeroException. it throws the exception OverflowException. // Max short value static short y = 32767. } } Output: When you run the program.WriteLine("Checked output value is: {0}". /) can produce results that are outside the range of possible values for the numeric type involved. It is seldom useful. // Throws the exception OverflowException } public static void Main() { Console.MethodUnCh()). All Rights Reserved. *. return z. You can debug the program or abort execution. // Using a checked expression public static int MyMethodCh() { int z = checked((short)(x + y)).cs . return z.1 checked and unchecked operators The arithmetic operators (+.Using the unchecked statement with constant expressions // Overflow is checked at compile time using System. // UCS.18. Study Notes. Thus. public int MethodUnCh() { // Unchecked statement: unchecked { int z = x * y. class UCS { const int x = 2147483647. The unchecked operator disables arithmetic checking at compile time.Chapter 2 C# Program Elements 2. The checked operator tells the runtime to generate an OverflowException if an integral expression exceeds the arithmetic limits of that type. When integer overflow occurs. C# gives you the choice of handling or ignoring overflow.Using checked expressions // The overflow of non-constant expressions is checked at run time using System. // Max int const int y = 2.
&&. Note that the assignment operator itself (=) cannot be overloaded. Element access. Cast operations. or associativity of an operator. ?:. Instead. Thus.19 Operator overloading All unary and binary operators have predefined implementations that are automatically available in any expression. new. implementations that produce results other than those that are intuitively expected are strongly discouraged. casting a long to an int) and are subject to checked or unchecked execution. user-defined indexing is supported through indexers. 34 Copyright Satish Talim 2001-2002. the corresponding assignment operator is also implicitly overloaded. This example shows how you can use operator overloading to create a complex number class Complex that defines complex addition. precedence. is not considered an overloadable operator. The overloadable unary operators are: + + ! * ~ / ++ % -& | true ^ false << >> == != > < >= <= The overloadable binary operators are: Only the operators listed above can be overloaded. In addition to the predefined implementations. always has the predefined precedence level and is always left associative. such as a[x]. the * operator is always a binary operator. user-defined implementations can be introduced by including operator declarations in classes and structs. All Rights Reserved.C# . User-defined operator declarations always require at least one of the parameters to be of the class or struct type that contains the operator declaration. . and is operators. are overloaded by providing user-defined conversions. While it is possible for a user-defined operator to perform any computation it pleases. 2. sizeof. For example.cs // Complex. Example: Complex. For example. it is not possible to overload member access. an implementation of operator == should compare the two operands for equality and return an appropriate result. typeof. or the =.CONSOLE APPLICATIONS } } Output: Unchecked output value: -2 In addition to the arithmetic operators. For example. The program displays the imaginary and the real parts of the addition result. User-defined operator declarations cannot modify the syntax. it is not possible for a userdefined operator to have the same signature as a predefined operator.cs using System. ||. public class Complex { public int real = 0. Study Notes. integral-type to integral-type casts can cause overflow (for example. User-defined operator implementations always take precedence over predefined operator implementations: Only when no applicable user-defined operator implementations exist will the predefined operator implementations be considered. method invocation. In particular. an overload of operator * is also an overload of operator *=. When a binary operator is overloaded. such as (T)x. An assignment always performs a simple bit-wise copy of a value into a variable.
though there are some noteworthy additions and modifications.imaginary). } } shows a program that uses an if statement to write out two different messages depending on whether command-line arguments were provided or not.4).real = real.imaginary).WriteLine("No arguments were provided").imaginary = imaginary.Length == 0) Console.2 The switch statement A switch statement executes the statements that are associated with the value of a given expression. Complex num2 = new Complex(3. An if statement may optionally include an else clause that executes if the boolean expression is false.WriteLine("Real: {0}".real. the types that can be added (two Complex objects). // Line 16 Console. public Complex(int real. class Test { static void Main(string[] args) { if (args.3). 2. 35 . else Console.Chapter 2 C# Program Elements public int imaginary = 0.20. } public static void Main() { Complex num1 = new Complex(2. int imaginary) { this. 2. c1.1 The if statement An if statement selects a statement for execution based on the value of a boolean expression. // Line 27 } } Code Discussion: • • • Line 010 declares which operator to overload (+). The example using System.20.real + c2. Study Notes.20 Program Control C# borrows most of its statements directly from C and C++. } public static Complex operator +(Complex c1. Copyright Satish Talim 2001-2002.WriteLine("Arguments were provided"). // Line 26 Console. Line 016 adds two Complex objects (num1 and num2) through the overloaded plus operator declared on line 010. 2.WriteLine("Imaginary: {0}". Complex sum = num1 + num2. All Rights Reserved. sum.imaginary + c2. and the return type (Complex). or default of statements if no match exists. this. Complex c2) { //Line 10 return new Complex(c1. Lines 017 and 018 show on execution that the real and imaginary parts of objects num1 and num2 have been added to produce the sum values. sum.real). The example using System.
default: Console. The end of a case statement must explicitly state where to go next. (i. break.WriteLine("Bye.WriteLine("{0} arguments were provided"). case "quit": Console. } decide: Console. break. begin: Console. No code in the case) then you can fall through. default: Console. // switch with string type switch (myInput) { case "continue": goto begin. myInt).C# . Study Notes. and have to use a goto to jump to the next (or previous) case.WriteLine("Your number {0} is not between 1 and 3. break. break. 36 Copyright Satish Talim 2001-2002. myInt = Int32.". myInt).Write("Type \"continue\" to go on or \"quit\" to stop: "). } } } switches on the number of arguments provided. If you have adjacent case labels. myInput = Console.Length) { case 0: Console.").Write("Please enter a number between 1 and 3: "). Unlike Java.". break. You will get a compiler error instead. myInput = Console. // switch with integer type switch (myInt) { case 1: Console. case 3: Console. class SwitchSelect { public static void Main() { string myInput. you no longer fall through from case to case if you omit the break and there's code in the case. myInt).". case 1: Console. The switch expression must be an integer type (including char) or a string. Example: SwitchSelection. int myInt.cs using System.WriteLine("Your number is {0}.WriteLine("Your number is {0}. break.CONSOLE APPLICATIONS class Test { static void Main(string[] args) { switch (args. The case labels have to be constants. break.ReadLine().".ReadLine().e. case 2: Console.WriteLine("No arguments were provided").WriteLine("Your number is {0}. myInt).Parse(myInput). All Rights Reserved.WriteLine("One arguments was provided"). .
4. } return i.20. } } Copyright Satish Talim 2001-2002. 1})). All Rights Reserved.Chapter 2 C# Program Elements default: Console.WriteLine(i).20.WriteLine("Your input {0} is incorrect. int[] arr) { int i = 0. 2. The example using System. } } } 2.3 The while statement A while statement conditionally executes a statement zero or more times – as long as a boolean test is true. The example using System.Length) throw new ArgumentException(). Study Notes.ReadLine(). class Test { static int Find(int value.4 The do statement A do statement conditionally executes a statement one or more times. } while (s != "Exit"). do { s = Console. goto decide.20. repeatedly executes a statement and evaluates a sequence of iteration expressions. using System. i < 10. new int[] {5. 2. 2. } } uses a while statement to find the first occurrence of a value in an array. } static void Main() { Console.5 The for statement A for statement evaluates a sequence of initialization expressions and then. while (arr[i] != value) { if (++i > arr. while a condition is true.WriteLine(Find(3. myInput). } } reads from the console until the user types "Exit" and presses the enter key. 3. class Test { static void Main() { string s. 37 .". i++) Console. class Test { static void Main() { for (int i = 0.
cs using System.In has two methods for obtaining input. so this is a good starting point.In).3.Error) streams. Write() and WriteLine() have numerous overloads.21. class FE { static void Main() { int[] arr1 = new int[] {1. accessed through a reference variable.cs: using System. 38 Copyright Satish Talim 2001-2002.ReadLine() and Console. or null if no more lines are available.21 Console I/O Console I/O is provided by the System. so that you can easily output many different types of data. Read() returns a single character as an int. 2.1 Console Input Console. Write() outputs one or more values without a newline character.Out. i).C# . Single-dimensional arrays are the most common type. the number showing which argument is to be substituted: "The value is {0}" "{0} plus {1} = {2}" // use the first argument // use the first three arguments Note that Console. } } uses a foreach statement to iterate over the elements of an array.ReadLine() are equivalent. standard output (Console. If you want to produce formatted output. 2. Formats contain both static text.21.6 The foreach statement A foreach statement lets you iterate over the elements in arrays and collections. foreach (int i in arr1) Console.In.4.20. Note that Console. a marker is a number in curly brackets.CONSOLE APPLICATIONS uses a for statement to write out the integer values 1 through 10.5}.22 Array types Arrays in C# are objects.WriteLine() are equivalent. 2. 2. The example FE. ReadLine() returns a string containing the next line of input.WriteLine() and Console.WriteLine("Value is {0}". Study Notes.Console class. All Rights Reserved. or –1 if no more characters are available.Out has two methods for writing output.2. In its simplest form. Arrays in C# may be singledimensional or multi-dimensional. plus markers that show where items from the argument list are to be substituted. 2. The example MyArray. . which gives you access to the standard input (Console.Out) and standard error (Console. you use a version of WriteLine() which takes a string containing a format and a variable number of objects. and how they are to be formatted.2 Console Output Console. WriteLine() does the same but appends a newline.
arr[i]).2. // "jagged" array: array of (array of int) int[][][] j3. int[. In rectangular arrays. 4. each of which can be of different length if desired.5}.Length. 30]. initializes the array elements. } } Copyright Satish Talim 2001-2002. 3}. 4. Arrays are reference types.] {{1. 6. C# supports two types of multidimensional arrays – rectangular and jagged. The example class Test { static void Main() { int[] a1 = new int[] {1.] a3. 3}.2. // any attempt to access an array outside the bounds // will result in a runtime error for (int i = 0. Array instances are actually created via array initializers and array creation expressions.. 2.] a2. and then prints each of them out.3. 2. 5. 2.WriteLine("arr[{0}] = {1}". i++) // index 0 for arrays arr[i] = i * i. The program output is: arr[0] arr[1] arr[2] arr[3] arr[4] = 16 = = = = 0 1 4 9 The type int[] used in the previous example is an array type. Array types are written using a non-array-type followed by one or more rank specifiers. // array of (array of (array of int)) } } shows a variety of local variable declarations that use array types with int as the element type.5}. A jagged array is simply an array of one-dimensional arrays.. 5. 2. i. every row is the same length. Study Notes. 39 . j2[2] = new int[] {1. // or // int[] arr = {1. // 2-dimensional array of int int[. j2[0] = new int[] {1. can't be changed for (int i = 0. // we can also do something like this // int[] arr = new int[] {1. i < arr. {4. The example: class Test { static void Main() { int[] a1.Chapter 2 C# Program Elements class MyArray { static void Main() { int[] arr = new int[5]. } } creates a single-dimensional array of int values. int[. // single-dimensional array of int int[. 9}.4. 7. j2[1] = new int[] {1. 2. 20. 6}.] a3 = new int[10. and so the declaration of an array variable merely sets aside space for the reference to the array.] a2 = new int[. i < arr. // size fixed to 5. i++) Console.3. 3.Length. 3. 6}}. 8. // 3-dimensional array of int int[][] j2.4. 3}. 5. int[][] j2 = new int[3][]. All Rights Reserved.
} } shows that the same array initializer can be used for several different array types. and the length of j2[2] is 9. has an array type of int[. The example can be rewritten to explicitly specify the type of array being created. 3} is used determines the type of the array being initialized. All Rights Reserved. Rectangular arrays always have a rectangular shape. Specifically. 2. 3}. Study Notes. It is important to note that the context in which an array initializer such as {1. The example gives each of the int[] arrays a different length. Each of these int[] variables can be initialized individually. For example. Given the length of each dimension of the array. For local variable and field declarations. and 30 respectively. In contrast. without any change in program semantics. the length of j2[0] is 3. a2 and a3 denote rectangular arrays. For instance the declaration int[. 3}). as in class Test { static void F(int[] arr) {} static void Main() { F(new int[] {1. It is important to note that the element type and number of dimensions are part of an array's type. or an "array of arrays". and the variable j2 denotes a jagged array. The example class Test { static void F(int[] arr) {} static void Main() { F({1. 3} is not a valid expression. 2. the example int[] a1 = new int[] {1. Because context is required to determine the type of an array initializer. . it is not possible to use an array initializer in an expression context. 3}. the variable j2 denotes a "jagged" array. 3}). 2. int[] b = {1. long[] c = {1. 40 Copyright Satish Talim 2001-2002. 20. and this allows the array to take on a jagged shape. The variables a1. 20. 2. can be shortened to int[] a1 = {1. 3}. a shorthand form is permitted so that it is not necessary to re-state the array type. the length of j2[1] is 6. or a single-dimensional array of type int[].] a3 = new int[10. the length of a3's three dimensions are 10.C# . and it is easy to see that this array contains 10*20*30 elements. j2 denotes an array of an array of int.CONSOLE APPLICATIONS shows a variety of array creation expressions. its rectangular shape is clear. 30]. as the length of each dimension is specified in the array creation expression rather than in the array type. The example class Test { static void Main() { short[] a = {1. 2. This split is made clear in the language syntax. 30]. It should be no surprise that these terms are based on the shapes of the arrays. For instance. 2. 3}. } } is not valid because the array initializer {1. 2. 2. 20. but that the length of each dimension is not part of the array's type. 3}.. 2. Specifically.] and an array creation expression of new int[10..
Array is the base type of all array types. such as methods for sorting. int LengthOfNumbers = numbers. Study Notes. i++) for (int j=0. You can use the properties. // second row has 3 elements arr4[2] = new int[4].WriteLine("{0}". Getting Command-Line Input: Example: NamedWelcome. // Program start class class NamedWelcome { Copyright Satish Talim 2001-2002.5]. j<5.cs //JA. i<5. and copying arrays.Array class provides many other useful methods/properties. 41 . 4. and other class members that System. // create a 2D array with implied size of 3 by 2 int[. // first row {4. // third row has 4 elements arr4[3] = new int[10]. class JA { public static void Main() { int[][] arr4. The following code assigns the length of the numbers array. arr3[1. // create 5 by 5 array for (int i=0. return 0. 3. The System.1]).cs // Namespace Declaration using System. class RA { public static int Main() { int[.Chapter 2 C# Program Elements } } Example RA. Console.Length.cs using System. j++) arr2[i.Array has.] arr3 = new int[. to a variable called LengthOfNumbers: int[] numbers = {1. arr2[2. } } Example JA. // assign an element } } In C#.cs // RA. 2. which is 5.cs using System. arrays are actually objects.j] = i*j.WriteLine("{0}". All Rights Reserved.8} // third row }. An example of this would be using the Length property to get the length of an array. // 2D jagged array of ints arr4 = new int[4][]. searching. // 2D rectangular array of ints arr2 = new int[5. System.2]).] { {1. // second row {7.2}.] arr2. // first row has 5 elements arr4[1] = new int[3]. // four rows in this array arr4[0] = new int[5]. // last row has 10 elements arr4[1][1] = 3. 5}. // write out some elements Console.5}.
// Main begins program execution. public static void Main(string[] args) { // Write to console Console.WriteLine("Hello, {0}!", args[0]); Console.WriteLine("Welcome to the CSharp Seminar!"); } } Remember to add your name to the command-line, i.e. "NamedWelcome Satish". If you don't, your program will crash. You'll notice an entry in the "Main" method's parameter list. The parameter name is "args". It's what you use to refer to the parameter later in your program. The "string[]" expression defines the Type of parameter that "args" is. The "string" Type holds characters. These characters could form a single word, or multiple words. The "[]", square brackets denote an Array, which is like a list. Therefore, the Type of the "args" parameter, is a list of words from the command-line. You'll also notice an additional "Console.WriteLine(...)" statement within the "Main" method. The argument list within this statement is different from before. It has a formatted string with a "{0}" parameter embedded in it. The first parameter in a formatted string begins at number 0, the second is 1, and so on. The "{0}" parameter means that the next argument following the end quote will determine what goes in that position. The next argument following the end quote is the "args[0]" argument, which refers to the first string in the "args" array. The first element of an Array is number 0, the second is number 1, and so on. For example, if we write "NamedWelcome Satish" on the command-line, the value of "args[0]" would be "Satish". Now we'll get back to the embedded "{0}" parameter in the formatted string. Since "args[0]" is the first argument, after the formatted string, of the "Console.WriteLine()" statement, it's value will be placed into the first embedded parameter of the formatted string. When this command is executed, the value of "args[0]", which is "Satish" will replace "{0}" in the formatted string. Upon execution of the command-line with "NamedWelcome Satish", the output will be as follows:
Hello, Satish! Welcome to the CSharp Seminar!
Another way to provide input to a program (InteractiveWelcome.cs) is via the console. The next program shows how to obtain interactive input from the user. //InteractiveWelcome.cs // Namespace Declaration using System; // Program start class class InteractiveWelcome { // Main begins program execution. public static void Main() { // Write to console/get input Console.Write("What is your name?: "); Console.Write("Hello, {0}! ", Console.ReadLine()); Console.WriteLine("Welcome to the CSharp Seminar!"); } } This time, the "Main" method doesn't have any parameters. However, there are now three statements and the first two are different from the third. They are "Console.Write(...)" instead of "Console.WriteLine(...)". The difference is that the "Console.Write(...)" statement writes to the console and stops on the same line, but the "Console.WriteLine(...)" goes to the next line after writing to the console. The first statement simply writes "What is your name?: " to the console. 42
Copyright Satish Talim 2001-2002, Study Notes. All Rights Reserved.
Chapter 2
C# Program Elements
The second statement doesn't write anything until its arguments are properly evaluated. The first argument after the formatted string is "Console.ReadLine()". This causes the program to wait for user input at the console, followed by a Return or Enter. The return value from this method replaces the "{0}" parameter of the formatted string and is written to the console. The last statement writes to the console as described earlier. Upon execution of the commandline with "InteractiveWelcome", the output will be as follows: What is your Name? <type your name here> Welcome to the CSharp Seminar! Hello, <your name here>!
2.23 Calling methods – ref / out
Methods are extremely useful because they allow you to separate your logic into different units. Defining and calling methods in C# closely follows the Java model. The structure of a method is as follows: attributes modifiers return-type method-name ( parameters ) { statements } We defer discussion of attributes and modifiers. The return-type can be any C# type, if the method does not return a value, its return type is void. It can be assigned to a variable for use later in the program or the return value can be ignored. The method name is a unique identifier for what you wish to call a method. To promote understanding of your code, a method name should be meaningful and associated with the task the method performs. Parameters allow you to pass information to and from a method. If there are no parameters, the method name is followed by empty parentheses. They are surrounded by parenthesis. Statements within the curly braces carry out the functionality of the method. As in Java, you are free to ignore the return value of a method call. What if you want to pass a value type into a method and modify it? C# provides two ways of doing this using the ref and out keywords. The ref keyword tells the C# compiler that the arguments being passed point to the same memory as the variables in the calling code. That way, if the called method modifies these values and then returns, the calling code's variables will have been modified. When you use the ref keyword, you must initialize the passed arguments before calling the method. See the example RefTest.cs below. using System; public class RefTest { int p = 3; public RefTest() { refMethod(ref p); } public static void Main(string[] args) { RefTest t = new RefTest(); Console.WriteLine(t.p); } void refMethod(ref int n) { n += 3; } } The only difference between the ref keyword and the out keyword is that the out keyword doesn't require the calling code to initialize the passed arguments first. See the example OutTest.cs below. using System; public class OutTest { 43
Copyright Satish Talim 2001-2002, Study Notes. All Rights Reserved.
int p; public OutTest() { outMethod(out p); } public static void Main(string[] args) { OutTest t = new OutTest(); Console.WriteLine(t.p); } void outMethod(out int n) { n = 3; } } We will now use the ref keyword on a string variable and modify the previous RefTest.cs program and call it RefTest2.cs using System; public class RefTest2 { string s = "Satish"; public RefTest2() { refMethod(ref s); } public static void Main(String[] args) { RefTest2 t = new RefTest2(); Console.WriteLine(t.s); } void refMethod(ref string s) { s = "Talim"; } } The output is Talim, a expected. If we now remove the ref keyword in the above program and call the new program RefTest3.cs using System; public class RefTest3 { string s = "Satish"; public RefTest3() { refMethod(s); } public static void Main(String[] args) { RefTest3 t = new RefTest3(); Console.WriteLine(t.s); } void refMethod(string s) { s = "Talim"; } } The output is Satish. Therefore, if a parameter is declared for a method without ref or out, the parameter can have a value associated with it. That value can be changed in the method, but the changed value will not be retained when control passes back to the calling procedure. By using a method parameter keyword (ref, out), you can change this behavior. 2.23.1 Method Overloading C# allows you to declare more than one method with the same name. This is called overloading. Overloaded methods must differ in the number and/or type of arguments they take. The return type does not play any part in the overload resolution, since it's always possible to call a method without using the return value.
Copyright Satish Talim 2001-2002, Study Notes. All Rights Reserved.
10). } public void WriteEntry(int resourceId) { Console. The DrawLine method tells the C# compiler that it can take a variable number of Point objects. or something derived from System. chart.Exception object.x = x.y). public int y. MyPoint p3 = new MyPoint(5.y = y. } } } class VarArgsApp { public static void Main() { MyPoint p1 = new MyPoint(5. this. Copyright Satish Talim 2001-2002. i++) { Console. class Log { public Log(string fileName) { // Open fileName and seek to end. Study Notes.WriteLine ("Retrieve string using resource id and write to log"). } public void WriteEntry(string entry) { Console. 20). MyPoint p2 = new MyPoint(5.WriteLine("{0}. Chart chart = new Chart(). int y) { this. p2. the method then uses a simple for loop to iterate through the Point objects that are passed.x.24 Handling Exceptions C# supports exceptions in very much the same way as Java. p[i]. Code that may give rise to exceptions is enclosed in a try block.DrawLine(p1. In C#.Chapter 2 C# Program Elements using System. printing each one.GetLength(0). } } class Chart { public void DrawLine(params MyPoint[] p) { Console. class MyPoint { public int x. 45 . i < p.Exception. All Rights Reserved. // VarArgsApp. which is followed by one or more catch blocks. 15). 2.WriteLine("\nThis method would print a line " + "along the following points:"). } } GetLength(0) returns the number of elements in the first dimension of the Array.2 Variable Method Parameters . for (int i = 0.params You can specify a variable number of method parameters by using the params keyword and by specifying an array in the method's argument list. p[i]. At run time. } } 2. {1}".23. public MyPoint(int x. p3).WriteLine(entry). what you throw has to be a System.cs using System.
cs using System. makes no difference to the output: using System. try { Console. and can be written like this: try { // code which may fail } catch { // handle error } A try block can also have a finally block associated with it. double result = a/b. These blocks don't specify an exception variable. double b = 0. or a return.NET framework includes built-in exception handlers for common exception types such as divide-by-zero exception. double b = 0. whether it is due to normal termination. By not requiring explicit exception declarations in method signatures.0. return or goto. The point to note for Java programmers is: Not only is the argument to catch optional.WriteLine("Result is {0}". A finally block can occur with or without catch blocks. result). C# values short-term programmer convenience over program safety and correctness. public class DBZ2 { public static void Main(string[] args) { double a = 15.cs as shown below. If a finally block exists. in the source file DBZ.0. it will be executed before the try block is completed. result). } } You will get the output: Result is Infinity Modifying the program to DBZ2. and after any possible exceptions are caught in the catch clause.CONSOLE APPLICATIONS Beta 1 of the .0. Console. double result = a/b. It is an error to transfer control out of a finally block using break. but the entire catch clause itself is optional. } } } A catch block which will catch any exception is called a general catch clause. Try this code. } catch (Exception e) { Console. This will happen no matter how control leaves the try. a break or continue (or goto) statement. .C# . Study Notes. C# has no throws keyword. to an exception occurring. All Rights Reserved.0. Also. public class DBZ { public static void Main(string[] args) { double a = 15. continue. public void ReadFile() { try { // code which may fail } finally { // close the file } } 46 Copyright Satish Talim 2001-2002.WriteLine("Result is {0}".WriteLine("Exception occured: " + e).
} } while(!finished). s = Console. } catch (VowelException x) { Console. } catch (ExitException z) { Console.StackTrace). finished = true.cs below: using class class class System.WriteLine("BlankException occured: " + y).WriteLine("This is the finally clause. All Rights Reserved. String s. } // Note unlike Java. . switch (c) { case 'A': case 'a': case 'E': case 'e': case 'I': case 'i': case 'O': case 'o': case 'U': case 'u': throw (new VowelException()). } } } 47 Copyright Satish Talim 2001-2002. do { try { processUserInput(). VowelException : Exception {} BlankException : Exception {} ExitException : Exception {} class MyException { public static void Main(String[] args) { bool finished = false. You can make up your own exception classes by deriving from one of the various exception classes existing. Study Notes.WriteLine("Trace: " + z.Write("Enter a character: "). // Using StackTrace property of Exception class Console. char c = s[0]. } catch (BlankException y) { Console.ReadLine(). See MyException.WriteLine("ExitException occured: " + z).WriteLine("VowelException occured: " + x)."). case 'X': throw (new ExitException()). } finally { Console.Chapter 2 C# Program Elements CoreException is C#'s equivalent of Error in Java and you are not expected to handle it. we do not declare that the method // processUserInput() throws … exceptions static void processUserInput() { Console. case ' ': throw (new BlankException()).
Just defining the Namespace keyword above the class definition is enough. one point of caution. Hence. while Folders are used to just manage and logically arrange Files. I am using third party components.Discount. In the same way. Namespaces can also contain many other Namespaces and classes.NET Platform with the use of Namespaces.It.Class relationship used in Java.Introduction { public class HelloMessage { public string GetMessage() { return "Hello. What should we call this namespace? By convention. However. When a language develops.C# . I am developer who is writing up a shopping cart class. the department names as well as the team names should be used . world" program. I purchase two such components. //Which component does the compiler here refer to? Reseller discount or Wholesaler Discount? int discount = d1. Classes actually contain data and Namespaces are used to logically arrange classes. All these parties try to give meaningful names to their classes like the Math class.CONSOLE APPLICATIONS 2. Microsoft is encouraging companies to use their full names to develop components and not abbreviations. we presented a "Hello. The problem arises when both these components have a class called Discount. namespace JavaTech. also if possible.CSharp. you DO NOT need to create folders to store classes into a Namespace. by providing an extra level of naming beyond the class name. AmericanBusinessCompany. a company like American Business Company should use its full name. Earlier.Discount instead of using ABC. Just consider an example. We'll put our class in a namespace named JavaTech.Discount class? Which will lead to the problem that we faced earlier! This is very true since Namespaces only help to extend the name of a class to make it unique. The relationship between Namespaces and classes can directly be compared to the relationship between Files and Folders.DotNet. Therefore.Introduction. even if they used namespaces their full names would be XYZ. world". C# programs are organized using namespaces. Again you might say that there are two companies whose abbreviated names might be XYZ and if both companies develop the Discount class then. The Dot '. To use namespaces all you have to do is place the namespace definition above your class definition.' is used to denote and access classes within the namespace. one calculates discount rates for retail customers and other calculates discount rates for wholesale customers. We as developers end up in misery due to this. Now in my shopping cart class how do I use both these classes unambiguously? eg.CSharp.Cal(45.cs) in two pieces: a HelloMessage component that provides messages and a console application that displays messages. Study Notes. Namespaces provide a way to group classes. All Rights Reserved. Discount d1 = new Discount(). Folders can also contain many files and sub folders within them. Files actually contain data. we'll provide a HelloMessage class in a namespace.25 Namespaces Namespaces are a great way of categorizing your types and classes to avoid name collisions. We'll now rewrite this program (as HelloMsg. many third party components are available. First. developers put all of their classes in a namespace that represents their company or organization. .78) . 48 Copyright Satish Talim 2001-2002. This will help to solve the naming conflict. or InterestCalculator etc. This concept may seem similar to the Package . Some compilers will complain others might compile and use any of the components as they wish! This problem has been identified and solved by Microsoft on the .
CSharp. unions. 49 . System. class Hello { static void Main() { MessageSource m = new MessageSource().Chapter 2 C# Program Elements } } } Namespaces are hierarchical.GetMessage()). We could just use the fully qualified name for the class – JavaTech.. Only public members are allowed in a namespace.CSharp. class Hello { static void Main() { HelloMessage m = new HelloMessage(). Namespace elements cannot be explicitly declared as private or protected.WriteLine(m. Such aliases can be useful in situation in which name collisions occur between two libraries. and delegates.CSharp. interfaces.HelloMessage. the compiler only accepts classes.Introduction is actually shorthand for defining a namespace named JavaTech that contains a namespace named CSharp that itself contains a namespace named Introduction.CSharp. System.. Study Notes.Introduction. which makes it possible to use all of the types in a namespace without qualification.. and the name JavaTech.Console.Introduction. Internal is the default.} } } Next. structs.GetMessage()). using JavaTech.WriteLine(m. All Rights Reserved.HelloMessage. we'll write a console application that uses the HelloMessage class. Our example can be rewritten using aliases as: using MessageSource = JavaTech.HelloMessage – but this name is quite long and unwieldy.CSharp. An easier way is to use a "using" directive. The following sample generates error CS0116: Copyright Satish Talim 2001-2002. The following sample generates error CS1527: namespace bad { private class foo1 {} // CS1527 protected class foo2 {} // CS1527 class foo3 { // allowed. The public keyword must be explicitly specified.Console.Introduction. or when a small number of types from a much larger namespace are being used. } } Note that the two occurrences of HelloMessage JavaTech. as in: namespace JavaTech { namespace Csharp { namespace Introduction {. are shorthand for C# also enables the definition and use of aliases. enums. This is internal static void Main() {} } } Inside a namespace.Introduction. } } Namespaces are often related to assemblies (which we will cover later).
you need to ensure separately that the compiler knows which assemblies to look in for the classes (/r switch in the compiler). followed by using or viceversa.WriteLine("GTst called. 50 Copyright Satish Talim 2001-2002. you can write code like the following: using output = System. The only purpose of the using command in this context is to save you typing and make your code simpler. } } The using keyword has two uses: • • Create an alias for a namespace (a using alias).WriteLine("Hello. Any identifier in the global namespace is available for use in a named namespace.cs using System. An example of this will make it clear. All Rights Reserved.C# .NET hierarchy. Permit the use of types in a namespace. sometimes called the global namespace. Java programmers should note that.. a default namespace is created. // CS0116 } Even if you do not explicitly declare a namespace. } } } class GTst { // this is in the global namespace public GTst() { Console. you do not have to qualify the use of a type in that namespace (a using directive).Console.cs. is present in every file. the following variant of the using directive does enable you to create aliases for classes: using alias = class Using this form of the using directive. cause any other code or libraries to be added to your project. class HelloWorld { public static void Main() { output. Although you can't specify a class in a using directive. namespace Test { public class Tst { public static void Main(string[] args) { GTst t = new GTst()."). This unnamed namespace. NSP. It does not. } } This gives you the flexibility to apply meaningful aliases to classes that are nested several layers deep in the . Namespaces implicitly have public access and this is not modifiable. compile and run: // NSP. Create a program.CONSOLE APPLICATIONS namespace x { int xx. for example. thus making your code a little easier to both write and maintain. Study Notes. If your code uses base classes. .. such that. we could use namespace first. World"). The using keyword cannot be declared inside a class.
dll. Go to the ".NA").Bank.dll is automatically referenced by the C# compiler. Example: For a simple Win Form application the compilation string is : csc /r:System. There is often a correspondence between namespaces and assemblies. then check out which Namespaces you have used.dll.dll. Once you have figured out the list of assemblies that you have to reference use the CSC C# compiler and while compiling add the reference to all the assemblies.Bank namespace would be built into an assembly called Finance.dll yourcodefile. To find this out start the Reference documentation from "Start -> Program Files -> Microsoft. A programmer employing your classes can get access to all the classes within a namespace through the using keyword.NET Framework Reference" section. such as graphics and sound files (assemblies are similar to jar files in Java).NET is the assembly. check all the Namespaces you have aliased and in which assembly (library dll) does it exist.cs 51 Copyright Satish Talim 2001-2002. . Let's see this in more detail: // NA.exe) file.WriteLine("Using A.dll. and any other files that the application needs to run. so that the classes in the say Finance. In order to create an assembly with a particular name.dll During compilation how can the compiler come to know which assemblies you have used and how can the compiler map the functions and class names? To enable the compiler to know where it can find the assembly which contains all the Namespaces and classes you have used.NET -> Reference Documentation".Drawing. On Windows systems. So how do you know which assemblies should you reference? A simple way would be to look at the using keyword. you have to reference those assemblies during compilation with the /Reference or /r compiler switch.cs The library file mscorlib.Banl. The reference documentation contains information about the assemblies/ libraries that contain the Namespace. assemblies can be an executable (.Microsoft. It may also contain resource files. An assembly consists of Intermediate Language code. the compiler cannot map the classes and Namespaces and a CS0234 Error "The type or Namespace "XXX" does not exist in the class or Namespace "XXX" is raised.cs /t:library /out:Finance. Study Notes. or some combination of these. If you fail to reference all the assemblies you have used.Win32. All Rights Reserved. you can define each of these classes in its own source code file.WinForms. If you're packaging several classes in a given namespace. One assembly can be split across more than one physical file. } } } // NB.26 Namespaces and Assemblies The basic element of packaging in . This is not mandatory. namespace A { public class NA { static void Main(String[] args) { } public NA() { Console. compile as follows: csc Bankstuff.Chapter 2 C# Program Elements 2.Interop.System. metadata that describes what is in the assembly.cs using System. a DLL.System.
C# program code is case sensitive and in free format. with no object files.exe will now run.exe).NET BETA 1 instructions for DLLs csc csc csc /t:library /t:library na. Microsoft . } static void Main() { } } } // Test.NB class"). The . Study Notes.dll test.cs /r:na.cs compile. I get an error: Test. rather than to native code which can be understood by the computer's processor. you will need Windows 2000.5.NET.cs(3. All Rights Reserved. 3. Base Class Library (BCL): C# itself does not include a class library.C# . using A. namespace A { public class NB { public NB() { Console. or when the application is run. IE 5. an optional Visual Studio .WriteLine("Using A. 4. Common Language Runtime (CLR): The code that runs within the CLR runs in an encapsulated and managed environment.cs.NET SDK (Beta 1). when I compile Test.NET and C# everything in the CTS is an object. NB o2 = new NB().7): error CS0234: The type or namespace name 'A' does not exist in the class or namespace '' To overcome this error use the .NET and an optional free C# IDE – Sharp Develop. } } Both NA.CONSOLE APPLICATIONS using System. 2. 52 Copyright Satish Talim 2001-2002. C# compiles straight from source code to executable (.cs and NB. All .cs nb.cs using System. separate from other processes on the machine.27 Summary of Key Concepts • • To build and run your C# programs. However. public class Test { public static void Main(string[] args) { NA o1 = new NA(). The BCL is shared between all programming languages on the . This IL code is then compiled to native code either when the application is installed.cs /r:nb. . • • • The name of the Csharp program can be anything you want.dll The program Test. Common Type System (CTS): In the world of .NET programs are compiled to an intermediate language (IL). 2.NET Framework consists of the: 1.
• • • • • • • Copyright Satish Talim 2001-2002. The process of turning a reference type back into a value type is called unboxing. the Main method has to be defined as follows: public static void Main(){ } The Main() method can be written as follows: 1. the namespace name does not have to map onto phyical directories. F and M prefixes denote types. C# supports two kinds of types: value types and reference types. • • • • • • • For Compiler Errors . Unlike Java. static members must be accessed through a class name. Class members (variables). Study Notes. 4. 53 . In Main() the first character is a capital M. const members are static. Every C# application must have a method named Main defined in one of its classes. unless you use the /doc:filename compiler option to tell it to generate the XML. the cast is necessary so that the compiler can verify that the cast is valid per the specified variable type. MyCounter) for namespaces.xml file . Observe the generated . as you can in Java.Chapter 2 • C# Program Elements A namespace in C# is somewhat similar to a package statement in Java. Therefore. but the whole package. static void Main(string[] args) – can take command line arguments. The /main compiler option specifies a class in which to look for a Main method. The readonly fields that we define are instance fields. static int Main() – it can return an int only. The process of turning a value type into a reference type is known as boxing. the compiler adds the full name of the member as a name attribute. However. Naming Guidelines: First alphabet of each word (eg. You can compile a program containing multiple Main() by using the /main:< className > switch with the C# compiler to specify which class's Main method is to be used. classes. C# supports three types of comments: //. public static void Main() 3. the first alphabet is lower-case and the start of each word is upper-case. It doesn't matter which class contains the method—you can have as many classes as you want in a given application—as long as one class has a method named Main.NET Framework SDK Documentation (which is installed with the . If you have zero or more than one Main() in a program. in C# you cannot import a single class. By default. Main() is declared inside a class or struct. In the case of unboxing. method arguments and interfaces (interfaces should start with I). static void Main() 2. you can expect compiler errors. A readonly modifier. specifies that the member can have its value set once only.You can search for the error code in the . Value types are allocated on the stack and Reference types are allocated on the heap. meaning that the user would have to instantiate the class to use the fields. an object could be cast to any type. You can't access them via an object. The T. /* and ///.All the members of the assembly are denoted by <member> tags. In C#. All Rights Reserved. However. using is very similar in concept to Java's import keyword.NET Framework SDK) for a more detailed description. and afterwards is read-only. methods. const members are calculated at compile-time. fields and members respectively. as is the case with the package statement in Java. The later will be treated as normal comments by the compiler.
It's possible to call Object methods on any value.CONSOLE APPLICATIONS • • • • All types – including value types – can be treated like objects and derive from Object. A jagged array is simply an array of one-dimensional arrays. Equals() is a very important method in Object class. you will get a compiler warning. Decimal division by zero always throws a DivideByZeroException. That way. The [] operator accesses individual characters of a string String literals are of type string and can be written in two forms. the calling code's variables will have been modified. which can be checked or unchecked. In C#. C# provides two ways of passing a value type into a method and modify it using the ref and out keywords. If you have adjacent case labels. No code in the case) then you can fall through. (i. In rectangular arrays. every row is the same length. if the called method modifies these values and then returns. you no longer fall through from case to case if you omit the break and there's code in the case.C# . • • • • • • • • • • • • • • 54 Copyright Satish Talim 2001-2002. Study Notes. When integer overflow occurs. All Rights Reserved. Common examples include financial calculations such as tax computations and currency conversions. the equality operators (== and !=) are overloaded to compare the values of string objects. quoted and @-quoted. an OverflowException is thrown. because floating-point types are based on IEEE 754 and so have provisions for representing infinity and NaN (Not a Number). When you use the ref keyword. each of the predefined types is a shorthand (alias) for a system-provided type. The default implementation of Equals supports reference equality only. each of which can be of different length if desired. and will be the same size on any system. Floating-point arithmetic overflow or division by zero never throws an exception. and have to use a goto to jump to the next (or previous) case. you must initialize the passed arguments before calling the method. If you declare a string as string str = new string("Hello"). . what happens depends on the execution context. C# supports two types of multidimensional arrays – rectangular and jagged.e. A foreach statement lets you iterate over the elements in arrays and collections. The decimal type provides 28 significant digits. You will get a compiler error instead. Unlike Java. many operators can be overloaded by the user. even values of "primitive" types such as int. but subclasses can override this method to support value equality instead. All the types have fixed sizes. string is a reference type. In a checked context. this method returns true if the two types are identical and have the same value. The + operator concatenates strings. In C#. The switch expression must be an integer type (including char) or a string. The ref keyword tells the C# compiler that the arguments being passed point to the same memory as the variables in the calling code. the most significant bits of the result are discarded and execution continues. The decimal type is appropriate for calculations in which rounding errors are unacceptable. not references. Integer division by zero always throws a DivideByZeroException. In an unchecked context. In the case of value types. The case labels have to be constants. Integer arithmetic overflow either throws an OverflowException or discards the most significant bits of the result. Decimal arithmetic overflow always throws an OverflowException.
You can specify a variable number of method parameters by using the params keyword and by specifying an array in the method's argument list. Even if you do not explicitly declare a namespace.Create an alias for a namespace or permit the use of types in a namespace. All Rights Reserved. but the entire catch clause itself is optional. it will be executed before the try block is completed. These blocks don't specify an exception variable. we could use namespace first. Namespace elements cannot be explicitly declared as private or protected. Namespaces implicitly have public access and this is not modifiable. • • • • • • • • • Copyright Satish Talim 2001-2002. Java programmers should note that. The argument to catch is optional. such that. you do not have to qualify the use of a type in that namespace.NET framework includes built-in exception handlers for common exception types such as divide-by-zero exception. Study Notes.Chapter 2 • • • C# Program Elements The only difference between the ref keyword and the out keyword is that the out keyword doesn't require the calling code to initialize the passed arguments first. Also. The using keyword has two uses . If a finally block exists. C# has no throws keyword. A finally block can occur with or without catch blocks. sometimes called the global namespace. and after any possible exceptions are caught in the catch clause. Only public members are allowed in a namespace (even internal and default are allowed). A catch block which will catch any exception is called a general catch clause. You can think of assemblies are something similar to jar files in Java. a default namespace is created. 55 . followed by using or viceversa. In C#. We are not allowed to use the same using directive more than once in our program ie. we cannot say: using System. Beta 1 of the .Exception object. what you throw has to be a System. is present in every file.Exception. or something derived from System. Any identifier in the global namespace is available for use in a named namespace. This unnamed namespace. using System.
an "A" is sent to your computer.C# . what every chip on the board does. For example. Attributes are the individual things that differentiate one object from another and determine the appearance. slowing down. Here's an example. especially if you're trying to create reusable objects. The significance of object-oriented technology is that it enables programmers to design software in much the same way that they perceive the real world. 3. Attributes of an object can also include information about its state. Study Notes. when you press the A key. and all you are interested in is how the unit interacts with each other. putting together the overall system is easy. Attributes are defined by variables. As the assembler of the overall system. you have a system in which all the units work together to create a larger system with which you can solve the problems you bought the computer in the first place. As the name implies. each of which has a specific role in the program and all of which can talk to each other in predefined ways. current pedal cadence. The procedures are also known as methods (function in C). objects are the key concept to understanding object-oriented technology. Will this video card fit into the slots on the motherboard and will this monitor work with this video card? Will each particular component speak the right commands to the other components it interacts with so that each part of the computer is understood by every other part? Once you know what the interactions are between the components and can match the interactions. and your bicycle. assemble an entire PC computer system from various components: a motherboard. changing gears). and with a little background and often some help. Using object-oriented programming. each of those components may be vastly complicated and engineered by different companies with different methods of design. However. number of gears) and behaviour (braking. Object Oriented Concepts Object-oriented technologies provide many benefits to software developers and their products. accelerating. a video card. 56 Copyright Satish Talim 2001-2002. a keyboard. a CPU chip. Thinking in objects is a dramatic departure from thinking procedurally. Software objects are often used to model real-world objects you find in everyday life. . Object-oriented concepts can be difficult to grasp and one's understanding of the concepts often slowly evolve over time. it is the expense of the learning curve. each component you use is a self-contained unit. You can walk into a computer store. Bicycles have attributes (current gear. you could have features for engine condition (off or on). If there's a downside. Object-Oriented Programming works in exactly the same way.CONSOLE APPLICATIONS 3. or how. your overall program is made up of lots of different self-contained components (objects).1 What is an Object? Objects are software bundles of data and related procedures that act on that data. your desk. you don't need to know how the component works. state or other qualities of that object. Ideally. your television set. and the process of designing objects is much more challenging than procedural design. a hard-disk. and so on. All Rights Reserved. for example. You can look around you now and see many examples of real-world objects: your dog. two wheels. Internally. when you finish assembling all the various self-contained units. These real-world objects share two characteristics: they all have attributes and they all have behaviour.
the bicycle is incapable of any activity. Everything that the software object knows (attribute) and can do (behaviour) is expressed by the variables and methods within that object. However. you can also use software objects to "objectify" abstract concepts. 3. the object's variables make up the center or nucleus of the object and the methods surround and hide the object's nucleus from other objects in the program. Thus. the implementation can change at any time without changing other parts of the program. how fast the rider is pedaling and how steep the hill is). its pedal cadence is 90 rpm. For example. and its current gear is the 5th gear. and it can't run. encapsulation is used to hide unimportant implementation details from other objects. Anything that an object does not know or cannot do is excluded from the object. "event" is a common object used in GUI window systems to represent the event when a user presses a mouse button or types a key on the keyboard. In addition. The bicycle is useful only when another object (you) interact with it (start pedaling).2 The Benefit of Encapsulation Encapsulating related variables and methods into a neat software bundle is a simple yet powerful idea that provides two primary benefits to software developers: • Modularity . Typically. When you want to change gears on your bicycle. there are no variables or methods for those states and behaviours. All Rights Reserved. A software object maintains its attributes in variables and implements its behaviour with methods. change the pedal cadence and change gears. As you can visualise. (The bike would not have a method for changing the speed of the bicycle as the bike's speed is really just a side-effect of what gear it's in. You can give your bicycle to someone else and it will still work. Study Notes. bark or fetch. A software object that modeled your real-world bicycle would have variables that indicated the bicycle's current state: its speed is 10 mph. Definition: An object is a software bundle of variables and related methods. You might want to represent a real-world bicycle as a software object within an electronic exercise bike. they too.the source code for an object can be written and maintained independently of the source code for other objects. However.3 What are Messages? Software objects interact and communicate with each other via messages. The software bicycle would also have methods to brake. Your bicycle hanging from a hook in the garage is just a bunch of titanium alloy and rubber. you just need to know which lever to move. • 3. It is through the interaction of these objects that programmers achieve higher order functionality and more complex behaviour. 57 .Chapter 3 Object Oriented Concepts Software objects are modeled after real world objects in that. have attributes and behaviour. your bicycle (probably) doesn't have a name.an object has a public interface which other objects can use to communicate with it. an object can be easily passed around in the system. the object can maintain private information and methods that can be changed at any time without affecting the other objects that depend on it. you don't need to know how the gear mechanism works. Information hiding . For example. Thus. Packaging an object's variables within the protective custody of its methods is called encapsulation. Copyright Satish Talim 2001-2002. A single object alone is generally not very useful and usually appears as a single component of a larger program or application that contains many other objects. You don't need to understand the gear mechanism on your bike in order to use it. by itself. These variables and methods are formally known as instance variables and instance methods to distinguish them from class variables and class methods. You can represent real-world objects in programs using software objects.
your bicycle is just one of many bicycles in the world. we say that your bicycle object is an instance of the class of objects known as bicycles.it would be very inefficient to produce a new blueprint for every individual bicycle they manufactured. For example. Three components comprise a message: • • • the object to whom the message is addressed (bicycle) the name of the method to perform (change gears) any parameters needed by the method (to a higher gear) These three components are enough information for the receiving object to perform the desired method. so message passing supports all possible interactions between objects. Software "blueprints" for objects are called classes. Using object-oriented terminology. The method's return type is the object's response to the message. object A sends a message to object B. . prototype or description that defines the variables and the methods common to all objects of a certain kind.for example. employee records. it's also possible to have many objects of the same kind that share characteristics: rectangles. manufacturers take advantage of the fact that bicycles share characteristics and they build many bicycles from the same blueprint . Class types support inheritance. classes are software blueprints for objects. No other information or context is required. Objects don't need to be in the same process or even on the same machine to send and receive messages back and forth to each other. and nested types. and events). you can take advantage of the fact that objects of the same kind are similar and you can create a blueprint for those objects. function members (methods. All bicycles have some attributes (current gear. a mechanism whereby derived classes can extend and specialize base classes. When object A wants object B to perform one of its methods.4 What are Classes? A class is a blueprint. In object-oriented software. current cadence. indexers. brake) in common. you often have many objects of the same kind. The message parameters are actually the parameters to a method. This information is passed along with the message as parameters. Study Notes. Hence. fields. In the real world. and destructors). However.CONSOLE APPLICATIONS Software objects interact and communicate with each other by sending messages to each other. All Rights Reserved.C# . When building bicycles. constructors. Sometimes the receiving object needs more information so that it knows exactly what to do . when you want to change gears on your bicycle. Like the bicycle manufacturers. two wheels) and behaviour (change gears. a class is a data structure that contains data members (constants. Hence. each bicycle's attribute is independent of and can be different from other bicycles. Definition: A class is a blueprint or prototype that defines a group of data items called fields or variables and the associated functions or methods that perform operations on this data. video clips and so on. operators. The Benefit of Messages: Everything an object can do is expressed through its methods. 3. The Benefit of Classes: 58 Copyright Satish Talim 2001-2002. you have to indicate which gear you want. properties.
5 What is Inheritance? Classes inherit variables and methods from their superclass. Even if you don't know what a penny-farthing is. Inheritance provides a powerful and natural mechanism for organising and structuring software programs. Class declarations are used to define new reference types. but also for graphics and networking behaviour. For example. events. racing bikes and tandems are all different kinds of bicycles. instances of those classes are created and discarded as needed. fields. as a C# programmer. you would know that it had two wheels. a class's methods and variables don't exist yet. The differentiation between classes and objects is often the source of some confusion. 59 . mountain bikes.Chapter 3 Object Oriented Concepts Objects provide the benefit of modularity and information hiding.not only for basic programming tasks (classes to provide basic math functions. speed and the like. You must create an instance from the class before you can call the methods and before the variables can have any values. However. So far. constructors. an instance is its concrete representation. is to create the right set of classes to accomplish what your program needs to accomplish. The C# environment comes with a library of classes that implement a lot of the basic behaviour you need . An instance of a class is another word for an actual object. each subclass inherits methods from the superclass. In addition. Then. Similarly. Your task. Study Notes. If the class is the general representation of an object. racing bikes and tandems. In comparison. handle bars and pedals. When you write a C# program. Bicycle manufacturers reuse the same blueprint repeatedly to build many bicycles. You know a lot about an object by knowing its class. Class members can include constants. it's a little more difficult to differentiate classes and objects in software. You can send the object a message and it will respond by performing the method and perhaps modifying the values of the variables. This is partially because software objects are electronic models of real-world objects or abstract concepts in the first place. Software programmers use the same class repeatedly to create many objects. destructors. an object's methods and variables actually exist and you can use it. strings. In object-oriented terminology. but a class may implement multiple interfaces. indexers. it's obvious that classes are not themselves the objects that they describe . and so on). arrays. Each subclass inherits attribute (in the form of variable declarations) from the superclass.a blueprint of a bicycle is not a bicycle. if I told you it was a bicycle. methods. racing bikes and tandems share some attributes: cadence. mountain bikes. Generally speaking. Because each instance of a class can have different values for its variables. each variable is called an instance variable. In the real world. People use the term "object" inconsistently and use it to refer to both classes and instances. racing bikes and tandems share some methods: braking and changing pedaling speed. and nested type declaration. Mountain bikes. Copyright Satish Talim 2001-2002. Classes provide the benefit of reusability. you design and construct a set of classes. Mountain bikes. operators. 3. A class library is a set of classes. racing bikes and tandems are all subclasses of the bicycle class. All Rights Reserved. properties. when your program runs. C# supports single inheritance only. Object-oriented systems take this a step further and allow classes to be defined in terms of other classes. objects are defined in terms of classes. the bicycle class is the superclass of mountain bikes.
. The MotorCycle class you created. What if your class defines entirely new behaviour. Tandem bicycles have two seats and two sets of handle bars. Methods and variables are inherited down through the levels. With inheritance. • 60 Copyright Satish Talim 2001-2002. All Rights Reserved.the inheritance tree. C# automatically assumes that you are inheriting from Object. and isn't really a subclass of another class? Your class can also inherit directly from Object.C# .CONSOLE APPLICATIONS However. inherited from Object. Other programmers fill in the details with specialised subclasses. Subclasses can also override inherited methods and provide specialised implementations for those methods. you would override the "change gears" method so that the rider could actually use those new gears. which still allows it to fit neatly into the C# class hierarchy. The further down in the hierarchy a class appears. if you had a mountain bike with an extra set of gears. The Benefit of Inheritance: • Subclasses provide specialised behaviours from the basis of common elements provided by the superclass. all classes inherit from this one superclass. programmers can reuse the code in the superclass many times.e. some mountain bikes have an extra set of gears with a lower gear ratio. For example.implemented. it defines methods inherited by all the classes in the C# class hierarchy. subclasses are not limited to the state and behaviours provided to them by their superclass. Each C# class can have only one superclass i. if you create a class definition that doesn't indicate its superclass in the first line. single inheritance. Study Notes. Subclasses can add variables and methods to the ones they inherited from the superclass. Programmers can implement superclasses that define "generic" behaviours (called abstract classes). You can think of a class hierarchy as defining very abstract concepts at the top of the hierarchy and those ideas becoming more concrete the farther down the chain of superclasses you go. At the top of the C# class hierarchy is the class Object. the more specialised its behaviour. or class hierarchy. In fact. You are not limited to just one layer of inheritance . The essence of the superclass is defined and may be partially implemented but much of the class is left undefined and non. Object is the most general class in the hierarchy. can be as deep as needed.
followed by a class-body.cs using System.WriteLine("Engine is now ON"). Study Notes.WriteLine("Engine is Already ON"). '\0' for chars and null for references. Class methods are invoked by using the dot notation. m. followed by the keyword class and an identifier that names the class. m. make. 0 for numeric types. Class and Object 4. m. All declarations of data members and methods take place between the opening and closing curly brackets.opt A class-declaration consists of an optional set of attributes. m.colour = "black". 61 . followed by an optional set of classmodifiers. As in Java. public class MotorCycle { string colour.showAttr(). false for Boolean. A class-declaration is a type-declaration that declares a new class. The data members in C# classes are set to certain default values. optionally followed by a semicolon. } public static void Main(String[] args) { MotorCycle m = new MotorCycle().startEngine(). the classes are defined and implemented in the same place.cs using System. else Console. Some example classes: // MotorCycle.WriteLine("Colour is = " + colour).Chapter 4 Class and Object 4. } } //Account.1 Class Declaration The use of a class tells the compiler that it's a reference type. public MotorCycle() { Console.make = "Yamaha".WriteLine("Engine is Running").startEngine(). m.WriteLine("Make is = " + make). } } public void showAttr() { Console. Console. Console. } public void startEngine() { if (engineState) Console. if (engineState) Console. Copyright Satish Talim 2001-2002. bool engineState. class-declaration: attributesopt class-modifiersopt class identifier class-baseopt class-body . followed by an optional class-base specification. else { engineState = true. All Rights Reserved.WriteLine("Engine is Idle").WriteLine("Constructor called").
1. Study Notes. All Rights Reserved. } } public class Test { public static void Main(String[] args){ Account a1 = new Account(). internal. Account a2 = new Account(3000. It specifies that the class hides an inherited member by the same name.0) return false. else balance -=amt. return true. // data member public Account(){ balance = 0. . and private modifiers control the accessibility of the class. return true.0) return false. } public double QueryBalance(){ return balance. } public bool Deposit(double amt){ //deposit cash if (amt <= 0. 62 Copyright Satish Talim 2001-2002. The abstract and sealed modifiers are discussed in the following sections. The new modifier is only permitted on nested classes.CONSOLE APPLICATIONS class Account { private double balance.0. some of these modifiers may not be permitted. } public Account(double amt){ balance = amt. Depending on the context in which the class declaration occurs. } } public bool Withdraw(double amt){ //withdraw cash if ((balance-amt) < 0. protected. } } 4. else { balance += amt.1 Class modifiers A class-declaration may optionally include a sequence of class modifiers: class-modifier: new public protected internal private abstract sealed It is an error for the same modifier to appear multiple times in a class declaration.C# . The public.00).
• • When a non-abstract class is derived from an abstract class.1. An abstract class differs from a non-abstract class in the following ways: • An abstract class cannot be instantiated. the non-abstract class must include actual implementations of all inherited abstract methods and accessors. Since there are no outstanding abstract methods or accessors in C. Such implementations are provided by overriding the abstract methods and accessors.2 Sealed classes is abstract but it is The sealed modifier (similar to final in Java) is used to prevent derivation from a class. B: A { void G() {} override void F() { // actual implementation of F } } the abstract class A introduces an abstract method F. C is permitted (but not required) to be non-abstract. Study Notes.1. In the example abstract class public } abstract class public } class C: B { public A { abstract void F(). Class C overrides F and provides an actual implementation. An error occurs if a sealed class is specified as the base class of another class. While it is possible to have variables and values whose compile-time types are abstract. 4.1. such variables and values will necessarily either be null or contain references to instances of non-abstract classes derived from the abstract types. and it is an error to use the new operator on an abstract class. A sealed class cannot also be an abstract class. All Rights Reserved. You have to use the abstract modifier on the class as well.withdraw(double)' cntained in nonabstract class 'Account' The correct way to write is as follows: public abstract class Account { public abstract bool withdraw(double amt). CurrentAccount.1 Abstract classes The abstract modifier is used to indicate that a class is incomplete and intended only to be a base class of other classes. 63 . } When we compile this we get an error: Test.24): error CS0513: 'Account. Copyright Satish Talim 2001-2002.Chapter 4 Class and Object 4. You should note that the presence of an abstract method in a class doesn't implicitly make the class abstract. An abstract class cannot be sealed. Class B introduces an additional method G. but doesn't provide an implementation of F. An abstract class is permitted (but not required) to contain abstract methods and accessors. B must therefore also be declared abstract.1. } Implementing classes could be SavingsAccount. The following example illustrates this: public class Account { public abstract bool withdraw(double amt).cs(2.
public int y { get { return this.int y) { this.16).1. public class A { public class B { public class C { public C() { Console. Example: using System.CONSOLE APPLICATIONS The sealed modifier is primarily used to prevent unintended derivation.C# .Y = value. as you now know. All Rights Reserved. } set { this. } set { this. but it also enables certain run-time optimizations.cs using System.WriteLine("In C"). 64 Copyright Satish Talim 2001-2002.WriteLine("x = {0}. Study Notes. } private int X. public int x { get { return this.3 Inner Classes We can have inner classes as shown in the program IC.Y. . pt.C().x =x.x. Using the protected modifier would result in a warning from the compiler because of the fact that protected members are visible to derived classes and.B.1.y =y. } } private int Y. The sealed modifier cannot be used on methods.X =value. this.C c = new A. } } } static void Main() { A.B.X. Console. sealed classes don't have any derived classes. } } Note that we used the private access modifier on the internal class members X and Y. pt. sealed class MyPoint { public MyPoint(int x. y = {1}". 4. } } } class SealedApp { public static void Main() { MyPoint pt = new MyPoint(6.y).
it is an error for a public class to derive from a private or internal class (see example Assembly2. and B is said to be derived from A.1. the base classes of B are A and object. Study Notes. Except for class object. the complete set of classes upon which a class depends is the transitive closure of the directly depends on relationship. The object class has no direct base class and is the ultimate base class of all other classes. The direct base class of a class type must be at least as accessible as the class type itself. every class has exactly one direct base class. A class directly depends on its direct base class (if any) and directly depends on the class within which it is immediately nested (if any). its direct base class is implicitly object. A class inherits members from its direct base class. If a class declaration has no class-base.Chapter 4 Class and Object } } 4. Since A does not explicitly specify a direct base class. In the example: class A {} class B: A {} class A is said to be the direct base class of B.1 Base classes . which defines the direct base class of the class and the interfaces implemented by the class. it is an error for A to depend on B. When a class B derives from a class A. class-base: : : : class-type class-type interface-type-list . Given this definition. it specifies the direct base class of the class being declared. For example. the set of base classes is the transitive closure of the direct base class relationship. the example class A: B.2 Class base specification A class declaration may include a class-base specification.cs later on).2. interface-type-list interface-type-list: interface-type interface-type-list 4. All Rights Reserved. the direct base class is assumed to be object. 65 . In other words. Likewise. Referring to the example above.C {} class public } B: class A C { {} Copyright Satish Talim 2001-2002. interface-type When a class-type is included in the class-base.1. or if the class-base lists only interface types. The base classes of a class are the direct base class and its base classes. The example: class A: B {} class B: C {} class C: A {} is in error because the classes circularly depend on themselves.
Note that a class does not depend on the classes that are nested within it. File Assembly1. but A does not depend on B (since B is neither a base class nor an enclosing class of A). It is an error to reference a member with internal access outside the assembly within which it was defined. 4. Study Notes.C# . 4. the example is valid.1. a framework for building graphical user interfaces could provide Control and Form classes that cooperate using members with internal access. which circularly depends on A.1.1.2. Assembly1.2 Internal class Internal members are accessible only within files in the same assembly. which depends on B (its immediately enclosing class). they are not exposed to code that is using the framework. All Rights Reserved.cs: // compile with /target:library internal class BaseClass { public static int IntM = 0. in which case the class is said to implement the given interface types.CONSOLE APPLICATIONS is in error because A depends on B. cannot derive from a sealed class class B is in error because it attempts to derive from the sealed class A. Since these members are internal. 66 Copyright Satish Talim 2001-2002. an attempt to access the member of the base class will produce an error. Thus. } } 4. The first file contains an internal base class. A common use of internal access is in component-based development because it enables a group of components to cooperate in a private manner without being exposed to the rest of the application code.2.3 Interface implementations A class-base specification may include a list of interface types. In the example class class } A B: A { {} B depends on A (because A is both its direct base class and its immediately enclosing class). In the example sealed class A {} class B: A {} // Error.cs and Assembly2.3 Class body The class-body of a class defines the members of the class. This example contains two files. } File Assembly2.C (its direct base class). In the second file. BaseClass.dll public class TestAccess { public static void Main() { // error. .cs: // compile with /target:exe /reference:Assembly1. For example.cs. BaseClass not visible outside assembly BaseClass myBase = new BaseClass(). It is not possible to derive from a sealed class.
Methods. Events. Operators.1 Class members Class and Object } The members of a class consist of the members introduced by its class-member-declarations and the members inherited from the direct base class. The function members of a class are the methods. A class-declaration creates a new declaration space. Destructors.1. Types. Static constructors. required to initialize instances of the class. Instance constructors. properties. can be performed by the class. indexers. which represent the types that. Properties. which implement the computations and actions that. which implement the actions. which define named attributes and the actions associated with reading and writing those attributes. Study Notes. required to initialize the class itself. which define the expression operators that. which implement the actions. are permanently discarded. which implement the actions to perform before instances of the class. operators.Chapter 4 class-body: { class-member-declarationsopt 4. which define notifications that. 67 . and the class-member-declarations immediately contained by the class-declaration introduce new members into this declaration space. class-member-declarations: class-member-declaration class-member-declarations class-member-declaration class-member-declaration: constant-declaration field-declaration method-declaration property-declaration event-declaration indexer-declaration operator-declaration constructor-declaration destructor-declaration static-constructor-declaration type-declaration The members of a class are divided into the following categories: • • • • • • • • • • • Constants.3. and destructors of the class. can be applied to instances of the class. Members that contain executable code are collectively known as the function members of the class. which represent constant values associated with the class. are generated by the class. All Rights Reserved. which permit instances of the class to be indexed in the same way as arrays. Fields. which are the variables of the class. are local to the class. constructors. Indexers. The following rules apply to class-member-declarations: Copyright Satish Talim 2001-2002.
the compiler will emit an error stating that you cannot define members with the same names as the enclosing type. the signature of a method must differ from the signatures of all other methods declared in the same class. and is usually used for initialisation. The signature of an operator specifically does not include the result type. • • • • The inherited members of a class are specifically not part of the declaration space of a class. a derived class is allowed to declare a member with the same name or signature as an inherited member (which in effect hides the inherited member). field. A constructor method has several characteristics: • • • It has the same name as the class name It has no return type It does not return any value. Study Notes. 4. The signature of an indexer must differ from the signatures of all other indexers declared in the same class. Java Programmers should note that: If you attempt to prefix a constructor with a type. } 68 Copyright Satish Talim 2001-2002. In the example VarTest.1. which is called when an object of a class type is constructed. All Rights Reserved. The signature of an operator must differ from the signatures of all other operators declared in the same class. and operators are characterized by their signatures: The signature of a method consists of the name of the method and the number. or type must differ from the names of all other members declared in the same class. Thus. constructors. All other members must have names that differ from the name of the immediately enclosing class. event. The signature of an operator consists of the name of the operator and the number and types of its formal parameters.C# . property. modifiers.3 Constructors A constructor is a method. The signature of an indexer specifically does not include the element type. The signature of a constructor consists of the number.3.1.CONSOLE APPLICATIONS • Constructors and destructors must have the same name as the immediately enclosing class. indexers. and types of its formal parameters.3. The name of a method must differ from the names of all other non-methods declared in the same class.cs: class VarTest { public VarTest() { } static void Main() { VarTest t. 4. The name of a constant. . In addition. The signature of an indexer consists of the number and types of its formal parameters. modifiers.2 Signature Methods. The signature of a method specifically does not include the return type. and types of its formal parameters.
How do I ensure that the desired class A constructor will be called? By explicitly telling the compiler which constructor I want called in the initalizer list. } } class B : A { public B() : base() { Console. class A has two constructors.WriteLine("B = {0}". This is useful when you have overloaded multiple constructors and want to make sure that a default constructor is always called.Object constructors—include an invocation of the base class's constructor immediately before the execution of the first line of the constructor.Chapter 4 Class and Object } If you compile the above program. is when I have two classes: A and B. one that takes no arguments and one that takes an int. } } class DerivedInitializer2App { public static void Main() { B b = new B(42). Copyright Satish Talim 2001-2002. All Rights Reserved. as below: using System. An initializer taking the form this(.WriteLine("A = {0}". Class B has one constructor that takes an int. This takes two forms: • • An initializer of the form base(.WriteLine("B").WriteLine("A"). } public A(int foo) { Console. 69 . Constructor Initializers: All C# object constructors—with the exception of the System.) enables the current class to call another constructor defined within itself. Study Notes. class A { public A() { Console. class A { public A() { Console. foo). the specific constructor implied by the form of the constructor called—to be called. } } Another example. using System. } } class B : A { public B(int foo) : base(foo) { Console. } } class BaseDefaultInitializerApp { public static void Main() { B b = new B().) enables the current class's base class constructor—that is. These constructor initializers enable you to specify which class and which constructor you want called. This time.WriteLine("A"). The problem arises in the construction of class B. foo). the C# compiler will warn you that the variable t has been declared but is never used in the application.
Also note that in the program there can be only one and only one Main() method. For example: • • • public class Trig { private Trig() {} // Prevent instantiation public const double PI = 3. If the direct base class does not have an accessible parameterless constructor. All classes can be public or otherwise.} public static double Cos(double x) {. The default constructor simply invokes the parameterless constructor of the direct base class. It's to be noted that: • Constructors are not inherited. } } You can use overloading to create several constructors for a class. it is not possible for other classes to derive from the class or create instances of the class (an exception being classes nested within the class)... it's advisable to write a default constructor for every class you write. Thus. 70 Copyright Satish Talim 2001-2002. Study Notes. When a class declares only private constructors. . As in Java. a default constructor is automatically provided.} } The Trig class provides a grouping of related methods and constants. If a class contains no constructor declarations.CONSOLE APPLICATIONS } } An example of this(.} public static double Tan(double x) {. } static void Main() { Tmp t = new Tmp(10).cs using System. class Tmp { public Tmp() { Console. If you write a class with only one constructor with parameters and try to create an object using a default constructor (not given by you) then the program does not compile.. a class has no other constructors than those that are actually declared in the class. then a default constructor need not be given. an error occurs.WriteLine("In Tmp with int"). All Rights Reserved.cs // ThisEx. Note that at least one private constructor must be declared to suppress the automatic generation of a default constructor (which always has public access). which one will get called depends on the arguments you give to new.WriteLine("In Tmp()"). } public Tmp(int i) : this() { Console.cs program below – Java programmers should note that the name of the program can be anything you want (in any case too). If you write a class with only one constructor with parameters and use it while creating an object. public static double Sin(double x) {. In the Tree. Constructors are usually public.C# . but can also be private or protected..14159265358979323846. Private constructors are commonly used in classes that contain only static members..) is now provided through the example ThisEx. It therefore declares a single private constructor. but is not intended to be instantiated..
} } Class and Object 4. The block of a static constructor declaration specifies the statements to execute in order to initialize the class.1. } } public class F : GF { // : is similar to extends in Java public F() { Console. and cannot be invoked explicitly.3. an error occurs.4 Calling Base Class Constructors public class CheckAccount : Account { public CheckAccount(double amt) : base(amt) { } } Here. Static constructors are not inherited. so the compiler can find and use it.e the constructor is called when the class is loaded. 71 . The exact timing and ordering of static constructor execution is not defined. The static constructor for a class is executed before any static member of the class is referenced. This corresponds exactly to the block of a static method with a void return type. } } public class S : F { public S() { Console. if a CheckAccount is created with a double being passed in to the constructor. Copyright Satish Talim 2001-2002.WriteLine("In S"). Account has a suitable constructor. In this case.Chapter 4 using System.1. public class GF { public GF() { Console. The static constructor for a class is executed before the static constructor of any of its derived classes are executed. Static constructors are declared using static-constructor-declarations: static-constructor-declaration: attributesopt static identifier ( ) block A static-constructor-declaration may include set of attributes. All Rights Reserved. 4. If any other name is specified. The compiler sees base(amt) as meaning. "Pass amt through to the base class constructor which takes one double as an argument. The identifier of a static-constructor-declarator must name the class in which the static constructor is declared. though several guarantees are provided: • • • • The static constructor for a class is executed before any instance of the class is created. } public static void Main(String[] args){ S son = new S(). Static constructors are invoked automatically.WriteLine("In F"). The static constructor for a class never executes more than once.5 Static Constructors Static constructors implement the actions required to initialize a class i.3.WriteLine("In GF"). that double is passed on to the base class constructor. Study Notes.
Study Notes. All Rights Reserved.sc using System. } public static void F() { Console.F"). class SC2 { static void Main() { Console. B.WriteLine("Init B").WriteLine("2"). The example SC2.F().F or the output: Init Init A. B.C# . } } class A { static A() { Console.F Init B.WriteLine("Init A").cs using System. The example SC1. Console.G().F B.F(). } } class A { static A() { Console.CONSOLE APPLICATIONS • • The static constructor cannot have parameters. . } } class B { static B() { Console. } } could produce either the output: Init A.F"). } public static void F() { Console.F because the exact ordering of static constructor execution is not defined.WriteLine("A. There's no such thing as a static destructor. } } class B: A { B A A B 72 Copyright Satish Talim 2001-2002.WriteLine("1"). class SC1 { static void Main() { A.WriteLine("Init A").WriteLine("B.
and there's a static constructor.WriteLine("Width = {0}. GraphicsPackage. In the example SRO.cs below.Chapter 4 Class and Object static B() { Console. Destructors implement the actions required to destruct instances of a class. } public static void G() { Console. All Rights Reserved.ScreenHeight). } } 4. The identifier of a destructor-declarator must name the class in which the destructor is declared. Destructors are declared using destructor-declarations: destructor-declaration: attributesopt ~ identifier ( ) block A destructor-declaration may include set of attributes. and have all the same disadvantages. the screen resolution fields are static and read-only.WriteLine("Init B"). Copyright Satish Talim 2001-2002. public static readonly int ScreenHeight. Study Notes. ScreenHeight = 768. 73 .WriteLine("B. } } class ReadOnlyApp { public static void Main() { Console.6 Destructors Destructors are also called a finalizer. If any other name is specified. The block of a destructor declaration specifies the statements to execute in order to initialize a new instance of the class. class GraphicsPackage { public static readonly int ScreenWidth. } } is guaranteed to produce the output: 1 Init A Init B B. an error occurs.3. C# destructors are very similar to Java's finalize() method. Height = {1}". This corresponds exactly to the block of an instance method with a void return type. which derives from it. ScreenWidth = 1024. static GraphicsPackage() ( // Code would be here to // calculate resolution.G"). GraphicsPackage.ScreenWidth.G 2 because the static constructor for the class A must execute before the static constructor of the class B.1. using System.
but all other members are. and cannot be invoked explicitly. . You can do this using the base keyword. the destructors in an inheritance chain are called in order. All Rights Reserved. public Test() { x = 0. A derived class extends its direct base class. This enables classes to exhibit polymorphic behavior wherein the actions performed by a function member invocation vary depending on the run-time type of the instance through which the function member is invoked. except for the constructors and destructors of the base class. Inheritance means that a class implicitly contains all members of its direct base class. properties. base is in many ways the equivalent of Java's super keyword. then C inherits the members declared in B as well as the members declared in A. • • • 4. but it cannot remove the definition of an inherited member. a class has no other destructors than those that are actually declared in the class. A class can declare virtual methods. and an implicit conversion exists from a derived class type to any of its base class types. Destructors are invoked automatically. A derived class can hide inherited members by declaring new members with the same name or signature.1.CONSOLE APPLICATIONS Destructors are not inherited. When an instance is destructed. and derived classes can override the implementation of these function members. An instance becomes eligible for destruction when it is no longer possible for any code to use the instance.3. Note however that hiding an inherited member does not remove the member—it merely makes the member inaccessible in the derived class. so there can only ever be one for a class. Thus. a reference to a derived class instance can be treated as a reference to a base class instance.1. However. A derived class can add new members to those it inherits. depending on their declared accessibility. regardless of their declared accessibility. Constructors and destructors are not inherited. Some important aspects of inheritance are: • • • Inheritance is transitive. The name of a class destructor is the class name preceded by a tilde (~).C# . Thus. An example: public class Test { private int x. 74 Copyright Satish Talim 2001-2002. They do not take any arguments. Study Notes. An instance of a class contains a copy of all instance fields declared in the class and its base classes.3. } ~Test() { // tidy up } } 4. from most derived to least derived. and B is derived from A. Execution of the destructor or destructors for the instance may occur at any time after the instance becomes eligible for destruction. If C is derived from B.Single A class inherits (use: instead of extends as in Java) the members of its direct base class. inherited members may not be accessible in a derived class.7 Inheritance . They are always public and have no return value.8 Accessing Base Class Members You may want to call a method in a class' base class from time to time. and indexers.
the declaration of the derived class member can include a new modifier to indicate that the derived member is intended to hide the base member. When this occurs. It also demonstrates how to access the hidden members of the base class by using the fully qualified names.x). public class MyBaseC { public static int x = 55. MyBaseC.10 The new modifier A class-member-declaration is permitted to declare a member with the same name or signature as an inherited member. 75 .1. declare it in the derived class using the same name. 4. Hiding an inherited member is not considered an error. which is a reference to the object that called the method.1.9 The this Reference A class method is always called in the context of some object. MyDerivedC. and a derived class.3. // Display the unhidden member y: Console. In the example below (MyDerivedC. and modify it with the new modifier. // Access the hidden value of x: Console. public void Invoke(). Suppose you wanted to know which object has called a method – you can get at this via the keyword this.WriteLine(x). a warning is issued to that effect.Chapter 4 Class and Object 4. To suppress the warning. public static int y = 22. This warning is suppressed by removing the new modifier. } Declaring a member with the name M in a derived class. However.cs) a base class. Consider the following class: public class MyBaseC { public int x. If a new modifier is included in a declaration that doesn't hide an inherited member. but it does cause the compiler to issue a warning. The example demonstrates the use of the new modifier. Study Notes. // Name hiding public static void Main() { // Display the overlapping value of x: Console. use the same field name x. the derived class member is said to hide the base class member. } } Copyright Satish Talim 2001-2002.WriteLine(y).WriteLine(MyBaseC. the field x will not be affected because it is not hidden by a similar name. It is an error to use the new and override modifiers in the same declaration. for example: public class MyDerivedC : MyBaseC { new public void Invoke(). Use the new modifier to explicitly hide a member inherited from a base class. To hide an inherited member. // The new modifier using System. All Rights Reserved. } will hide the method Invoke() in the base class. thus hiding the value of the inherited field. } public class MyDerivedC : MyBaseC { new public static int x = 100.3.
11 Casting between Types Assuming a base class named Employee and a derived class named ContractEmployee. Console. the following code works because there's always an implied upcast from a derived class to its base class: class Employee { } class ContractEmployee : Employee { } class CastExample1 { public static void Main () { Employee e = new ContractEmployee().1. compiles but throws a throws a class Employee { } class ContractEmployee : Employee { } class CastExample3 { public static void Main () { //Downcast will fail.C# .x'. . the program System. you don't have to worry about an exception being thrown.3. What will happen instead is that the result will be null. 4. // Won't compile. the following is illegal. In the next example.ToString()). but you will get the warning: SC0108: The keyword new is required on 'MyDerivedC. } } There is one other way of casting objects: using the as keyword.WriteLine("e = {0}". } } However. because the compiler cannot provide an implicit downcast: class Employee { } class ContractEmployee : Employee { } class CastExample2 { public static void Main () { ContractEmployee ce = new Employee(). The advantage to using this keyword instead of a cast is that if the cast is invalid. Study Notes. Here's an example: using System. } } Remember: a derived class can be used in place of its base class. the program will still compile and run.CONSOLE APPLICATIONS Output 100 55 22 If you remove the new modifier. 76 Copyright Satish Talim 2001-2002. class Employee { } class ContractEmployee : Employee { } class CastExample5 { public static void Main () { Employee e = new Employee(). e == null ? "null" : e. All Rights Reserved. ContractEmployee ce = (ContractEmployee)new Employee().InvalidCastException at run-time.x' because it hides inherited member 'MyBaseC.
c == null ? "null" : c. Console.house = true.Chapter 4 Class and Object ContractEmployee c = e as ContractEmployee. s. s. 77 . protected internal members are accessible only from derived classes within the same assembly. protected bool gold. s. public class Dad { public bool diamond.house). s.gold). s. you'll see the following result: c:>CastExample5 e = Employee c = null 4.diamond = true. the declaration defaults to private declared accessibility. } } If you run this example. protected members are accessible only from derived classes. internal members are accessible only from within the same assembly. All Rights Reserved.WriteLine("c = {0}".12 Access modifiers A class-member-declaration can have any one of the five possible types of declared accessibility: public. private members are accessible only from the class itself. // Here private is inaccessible Console.car = true. bool house. The examples below should illustrate the above: // Am. or private. internal. s.gold = true. The point to remember is that the access modifiers are associated with members or types (classes etc.WriteLine("house = {0}". gold = {1}". } class Son : Dad { public static void Main(String[] args) { Son s = new Son().) and are not allowed on namespaces. s.WriteLine("silver = {0}. Console. Each member of a class has a form of accessibility.WriteLine("diamond = {0}.diamond.silver. Except for the protected internal combination. car = {1}". it is an error to specify more than one access modifier. Console.car).3.1. internal bool car.ToString()). protected internal bool silver. When a class-member-declaration does not include any access modifiers. protected internal. Study Notes. protected. s. s. s. as under: • • • • • public members are available to all code.silver = true.cs using System. } } Copyright Satish Talim 2001-2002.
WriteLine("diamond = {0}.13 Restrictions on Using Accessibility Levels When you declare a type.. } class Friend { public static void Main(String[] args) { Dad d = new Dad(). remove the comments one at a time. and two nested classes.1. d.C# . For example. protected internal bool silver.3.silver.} public class MyClass: BaseClass {. d. it is essential to see if that type has to be at least as accessible as another member or type.diamond. car = {1}". Study Notes.14 Accessibility Domain The example AM3.1. The following declarations will result in a compiler error. d. bool house. Console.CONSOLE APPLICATIONS // AM2. // protected inaccessible d. Console. d. internal bool car. public class Dad { public bool diamond.3.gold). private static int myPrivateInt = 0. . protected bool gold. All Rights Reserved. T1.WriteLine("silver = {0}. In the Main method.cs contains a top-level type. public class M1 { public static int myPublicInt.car = true. the direct base class must be at least as accessible as the derived class. internal static int myInternalInt. 78 Copyright Satish Talim 2001-2002. a comment follows each statement to indicate the accessibility domain of each member.silver = true. namespace MyNameSpace { public class T1 { public static int myPublicInt. using System.house = true.cs using System. d.} // Error 4. d. Notice that the statements that attempt to reference the inaccessible members are commented out. } } 4. The classes contain fields with different declared accessibilities.car). internal static int myInternalInt = 0. If you want to see the compiler errors caused by referencing an inaccessible member. d.house). private static int myPrivateInt = 0... } private class M2 { public static int myPublicInt = 0.. // private inaccessible Console. M1 and M2. internal static int myInternalInt.diamond = true.gold = true. d. d.WriteLine("house = {0}". because the base class BaseClass is less accessible than MyClass: class BaseClass {. gold = {1}". private static int myPrivateInt = 0.
myInternalInt = 2. like this: // Create a savings account Account s1 = new CheckAccount(). so it looks at the reference s1.myPublicInt = 1.myInternalInt = 2. sees that it is of type Account. this works because a CheckAccount is an Account. // Error: inaccessible outside T1 // T1.M1. In this case. and so calls the Account. T1.myInternalInt = 2.M1. Copyright Satish Talim 2001-2002. as the compiler will insist on calling the one from Account. Suppose that we want to provide CheckAccount with its own version of Withdraw().myPrivateInt = 3.myPrivateInt = 3.myPublicInt = 1. } } } 4.M2.00). s1.M1. Study Notes. As I've already mentioned.myPublicInt = 1.Withdraw() method. if we try to use our new version through an Account reference. // Error: inaccessible outside T1 // T1. rather than from CheckAccount. public bool Withdraw(double amt) { if (balanceamt >= overdraftLimit) { base. Withdraw(2000.3. // Error: inaccessible outside M2 return 0. } else return false. We can code one up quite easily: public class CheckAccount : Account { private double overdraftLimit = 500. return true. // Access is unlimited T1. // T1. This problem arises because the compiler has to decide which method to call.Withdraw(amt). // Access is unlimited // Accessible only in current project // Error: inaccessible outside T1 // Access to the M1 fields: T1. } } There's a problem with this. // Error: inaccessible outside M1 // Access to the M2 fields: // T1. //CheckAccount : Account // Deposit and Withdraw s1.00).Deposit(15000.myPrivateInt = 3.Chapter 4 Class and Object } } public class MainClass { public static int Main() { // Access to T1 fields: T1. so we can do everything to a CheckAccount that we can to the base class. because CheckAccounts can have an overdraft limit.M2.15 Virtual methods One of the properties of inheritance is that we can use a derived class object through a base class reference. All Rights Reserved. // Accessible only in current project // T1.00. 79 .1. we are simply calling the Deposit() and Withdraw() methods that CheckAccount inherits from Account.M2.
.00)..WriteLine("Account. so it looks at the reference s1. // VO1.16 Override methods When you override a virtual method in a derived class. } } The output of the above is: Account...WriteLine("Account. allowing the dynamic type of the reference to be used.WriteLine("CheckAccount. sees that it is of type Account.. } } public class CheckAccount : Account { public void Withdraw(double amt) { Console.1. and so calls the Account.cs // VO.C# . As mentioned earlier. public override bool Withdraw(double amt) { if (balanceamt >= overdraftLimit) { base. All Rights Reserved. } } Let us wrap up this discussion with a consolidated example – VO. Study Notes. .cs using System.cs using System.. you can use the override keyword to signal that you are overriding a virtual method.3.Withdraw method called. You declare a method as virtual by using the virtual modifier in the base class: public class Account { Virtual public bool Withdraw(double amt) { } } 4.Withdraw(5000. s1. public class Account { public void Withdraw(double amt) { Console. } public static void Main(string[] args) { Account s1 = new CheckAccount()..cs. In the example VO1.Withdraw method called.00.").Withdraw(amt). return true.Withdraw method called. } else return false. public class CheckAcount : Account { private double overdraftLimit = 500. we use the virtual and override keywords to give a different ouput..").").Withdraw() method. public class Account { virtual public void Withdraw(double amt) { Console. the compiler has to decide which method to call.CONSOLE APPLICATIONS A virtual method is one where the decision on exactly which method to call is delayed until runtime. } } public class CheckAccount : Account { 80 Copyright Satish Talim 2001-2002.Withdraw method called.
as in Java. Remember that the virtual keyword must be used on the base class's method. Interfaces can contain methods.WriteLine("In B"). C# interfaces are very much similar to Java interfaces.MtdA(). c.. In C# we say : instead of implements.. 81 . All Rights Reserved. and events as members. The static and public modifiers are not permitted on interface methods.cs has the same method overridden multiple times. properties. using System.Withdraw method called.WriteLine("In C").00). fields (private data members). s1.Withdraw(5000. Study Notes. } } 4. class A { public virtual void MtdA() { Console. } } The output is: CheckAccount.WriteLine("CheckAccount.WriteLine("In A"). Interfaces are used to define a contract. This example VO2..Chapter 4 Class and Object public override void Withdraw(double amt) { Console. constructors and destructors or any type of static member. a class or struct that implements the interface must adhere to this contract. and the override keyword is used on the derived class's implementation of the method.2 Interfaces The interface keyword declares a reference type that has abstract members. } } class B : A { public override void MtdA() { Console.Withdraw method called. and the compiler will give you an error if you try to specify any other modifiers on interface members. They can't contain constants.. } } class Tmp { static void Main() { A c = new C(). } } class C : B { public override void MtdA() { Console. indexers. All the members of an interface are public by definition."). } public static void Main(string[] args) { Account s1 = new CheckAccount(). The example Copyright Satish Talim 2001-2002.
CONSOLE APPLICATIONS interface Iexample { string this[int index] { get. and a property P. IControl.Bind(Binder b) {. Explicit interface methods can only be called via the interface. the Paint method from the IControl interface and the Bind method from IDataBound interface are implemented using public members on the EditBox class. } public class EditBox: Control.. In the example below. string P { get. shows an interface that contains an indexer. the EditBox's implementation of the Paint method can be called only by casting to the IControl interface. public void Bind(Binder b) {.. } } public delegate void EventHandler(object sender. class Test { static void Main() { EditBox editbox = new EditBox(). editbox. Event e). C# provides an alternative way of implementing these methods that allows the implementing class to avoid having these members be public. the class EditBox derives from the class Control and implements both IControl and IDataBound.Paint(). set. Study Notes. the interface IComboBox inherits from both ITextBox and IListBox. void IDataBound. . All Rights Reserved. IdataBound { void IControl. // error: EditBox does not have a Paint method IControl control = editbox. IListBox {} Classes and structs can implement multiple interfaces. interface Icontrol { void Paint(). // calls EditBox's implementation of Paint } } 82 Copyright Satish Talim 2001-2002.. In the example below. } event EventHandler E. } interface ITextBox: Icontrol { void SetText(string text).C# . void F(int value). an event E. public class EditBox: IControl. interface IdataBound { void Bind(Binder b). For example. control. Interfaces may employ multiple inheritance.. } interface IComboBox: ITextBox. Interface members can be implemented by using a qualified name.} } Interface members implemented in this way are called "explicit interface member implementations" because each method explicitly designates the interface method being implemented.} } In the example above. For example.Paint and IDataBound. the EditBox class could instead be implemented by providing IControl. IdataBound { public void Paint().Bind methods. } interface IListBox: Icontrol { void SetItems(string[] items).Paint(). set. a method F.Paint().
however: structs are value types rather than reference types. y. It's quite possible to nest references to structs. You can also declare structs nested inside other structs. public Point() { x = 0. and can have the same kinds of members as classes. Structs differ from classes in several important ways.y = y. for the array. 4. Enums are typically used when for "multiple choice" scenarios. The list of similarities between classes and structs is long – structs can implement interfaces. Of course. the use of a struct rather than a class for a Point can make a large difference in the number of allocations. There is no substitute for careful data structure and algorithm design. Careful programmers can enhance performance through judicious use of structs. Study Notes. consisting of a number of elements (or members) of other types. class Point { public int x. this optimization can be mis-used. public Point(int x.Chapter 4 Class and Object 4. With Point implemented as a class. this. The Point instances are allocated in-line within the array. If you pass a struct to a function. the program instantiates 101 separate objects – one for the array and one each for the 100 elements. and can be accessed using a simple dot notation. i++) points[i] = new Point(i. Using structs instead of classes can also make your programs fatter and slower. } } If Point is instead implemented as a struct. i < 100. as the overhead of passing a struct instance by value is slower than passing an object instance by reference. } public Point(int x. } } class Test { static void Main() { Point[] points = new Point[100].4 Enums An enum type declaration defines a type name for a related group of symbolic constants (named integer constants). as in struct Point { public int x. int y) { this. The program below creates and initializes an array of 100 points. The variables which make up a struct are called its members (fields in C#). y = 0. the entire structure is copied onto the stack. and inheritance is not supported for structs.x = x.y = y. y.x = x. All Rights Reserved. } } then the test program instantiates just one object. For example. i*i). this. by default. and you can use the dot notation to access all levels within the nested structure. int y) { this. for (int i = 0. 83 . Struct values either are stored "on the stack" or "in-line". in which a Copyright Satish Talim 2001-2002.3 Structs A struct in C# is simply a composite data type.
The default type of the constants is int. For example.CONSOLE APPLICATIONS runtime decision is made from a number of options that are known at compile-time. } } } shows a Color enum and a method that uses this enum.Red: . Green } class Shape { public void Fill(Color color) { switch(color) { case Color. be attributed to the inclusion of properties as a first-class element. C# has this idea of accessing data members through get and set code built into the language. Each of the named constants has a value. ushort. case Color. All Rights Reserved. using a property looks like they are getting direct access to the data.. The signature of the Fill method makes it clear that the shape can be filled with one of the given colors. whereas in fact the compiler is mapping the call onto the get/set methods. break. but modern component-oriented programs tend to be chockfull of 84 Copyright Satish Talim 2001-2002. Tue.. The other example enum Color { Red. ulong. The success of rapid application development tools like Visual Basic can. and this allows them to focus on their own application logic rather than on the details of a component they happen to be using. the use of Color rather than int for a parameter type enables smart code editors to suggest Color values. The use of enums is superior to the use of integer constants – as is common in languages without enums – because the use of enums makes the code more readable and self-documenting. uint. to some extent.. any that you don't specify get a value one more than the proceeding constant.. On the face of it. VB developers can think of a property as being field-like. this difference might not seem like a big deal. Blue. default: break. the data members are normally private and access to them is through get and set (or accessor) methods. The drawback here is that one has to code the accessor methods for each data member and users have to remember to use them to access these data members. case Color. you can declare enum which use other integral types like – byte. in the form of properties. sbyte.C# ... Wed). long. 4. Study Notes. . break. which by default starts at zero and increases by one for each succeeding member. break.5 Properties In OO languages.Green: . enum Weekday : short (Mon. The self-documenting nature of the code also makes it possible for the development tool to assist with code writing and other "designer" activities. short.Blue: . You can give explicit values to any or all of the constants. The difference between using get/set methods and properties is that to a user.
Value++. // set // get // get & set You can omit either the set or get clause. higher level IAccount interface and an abstract class BankAccount to collect all features common to all bank accounts. 4. The get accessor is called when the property's value is read. The BankAccount class has account number and balance as instance variables. The SavingsAccount and CurrentAccount classes BonusSaverAccount is derived from SavingsAccount.Caption = "ABC". the BonusSaverAccount is designed with incentives to encourage faster savings accumulation. the Button class defines a Caption property. Properties can be inherited and you can use the abstract and virtual modifiers with them. public class Button: Control { private string caption. but the true value of properties shows itself is in their usage rather than in their declaration. so that derived classes can be required to implement their own versions of property methods.. In addition. } } } Properties that can be both read and written. However. the static modifier can be used to create properties that belong to classes as opposed to individual objects.Caption += "DEF". the new value for the property is given in an implicit value parameter (the value represents the value passed in from the user). Properties are defined in C# using property declaration syntax. string s = b. include both get and set accessors. Repaint(). Declaration of properties is relatively straightforward.SetValue(o.). The second part includes a get accessor and/or a set accessor. b.Chapter 4 Class and Object property reads and writes.GetValue() + 1). This means that properties don't have to be tied to a data member but can represent dynamic data. In the example below. The first part of the syntax looks quite similar to a field declaration. Both types of accounts earn interest. The SavingsAccount and BonusSaverAccount are both savings accounts through which you can make deposits and withdrawals. All Rights Reserved. o. In a set accessor. public string Caption { get { return caption. Copyright Satish Talim 2001-2002.Caption. It has the methods for deposit and withdrawal. the set accessor is called when the property's value is written.6 Assignment Use a generic. } set { caption = value. o. The Caption property can be read and written in the same way that fields can be read and written: Button b = new Button(). like the Caption property.) are clearly at a disadvantage compared to languages that feature field-like usage of properties (e. 85 . Languages with method-like usage of properties (e. there is a financial penalty every time a withdrawal is made from a BonusSaverAccount. b. You don't have to return the value of a variable in a get clause. but can use any code you like to calculate or obtain the value of the property. are derived from BankAccount and The CurrentAccount has an overridden withdrawal method that makes use of overdraft protection. Study Notes. but the BonusSaverAccount earns more.. Furthermore.g.g.
WriteLine("New Balance: " + balance).C# .WriteLine("Insufficient funds"). else { balance -= amount. 86 Copyright Satish Talim 2001-2002. public SavingsAccount (int accountNum. All Rights Reserved. balance = initialBal. } return result. double initialBal. private double balance.cs namespace BankSystem { using System. Console. } public virtual bool withdrawal(double amount) { bool result = false. } } public void deposit(double amount) { balance += amount.WriteLine("New Balance: " + balance).WriteLine("Amount: " + amount). double initialBal) { account = accountNum. Console. Console. Console. // Constructor: public BankAccount(int accountNum. bool withdrawal(double amount). public interface IAccount { // Two abstract methods: void deposit(double amount). } set { balance = value. } // Property implementation: public int Account { get { return account. . Study Notes. } } class SavingsAccount : BankAccount { protected double rate.WriteLine("Deposit into account " + account).WriteLine("Amount: " + amount). result = true. } } public double Balance { get { return balance. Console.CONSOLE APPLICATIONS // Assignment. Console.WriteLine("Withdrawal from Account " + account). if (amount > balance) Console. } public abstract class BankAccount : IAccount { // Fields: private int account. } set { account = value.
WriteLine("Interest added to account: " + Account). 0.00. Console. 10000.WriteLine("New balance: " + Balance). initialBal. } public virtual void addInterest() { // using the Property names: Account and Balance Balance += Balance * rate. public CurrentAccount(int accountNum.withdrawal(amount ... initialBal) { rate = interestRate.Chapter 4 Class and Object double interestRate) : base(accountNum.").").Balance)) Console.00. public BonusSaverAccount (int accountNum. Console. } } return result. if (! overdraft. SavingsAccount protection) : base(accountNum.00. Console.withdrawal(amount)) { Console. initialBal){ overdraft = protection.withdrawal(amount+PENALTY). } public override bool withdrawal(double amount) { bool result = false. result = true. CurrentAccount checking = new CurrentAccount(301. Copyright Satish Talim 2001-2002. 500.. return base. } } class BonusSaverAccount : SavingsAccount { private const int PENALTY = 25. interestRate){ } public override bool withdrawal(double amount) { Console. } public override void addInterest() { Balance += Balance * (rate + BONUSRATE).. double interestRate) : base(accountNum.WriteLine("Overdraft source insufficient. 0. 87 .04). 10000. double initialBal. else { Balance = 0. All Rights Reserved. double initialBal.WriteLine("New Balance on account " + Account + ": " + Balance).02).WriteLine("Penalty incurred: " + PENALTY). savings). if (! base.02.WriteLine("New balance: " + Balance). } } class CurrentAccount : BankAccount { private SavingsAccount overdraft.WriteLine("Interest added to account: " + Account). Console. Console. private const double BONUSRATE = 0.WriteLine("Using overdraft. } } class Assignment { public static void Main(String[] args) { SavingsAccount savings = new SavingsAccount(101. Study Notes. BonusSaverAccount bigSavings = new BonusSaverAccount(201.
savings.deposit(1000.00); bigSavings.deposit(1000.00); savings.withdrawal(500.00); bigSavings.withdrawal(500.00); checking.withdrawal(501.00); } } }
4.7 Summary of Key Concepts
• • • • • An abstract class indicates that the class is incomplete and intended only to be a base class of other classes. An abstract class cannot be instantiated and cannot be sealed. The sealed modifier (similar to final in Java) is used to prevent derivation from a class. The sealed modifier cannot be used on methods. The direct base class of a class type must be at least as accessible as the class type itself. Internal members are accessible only within files in the same assembly. Constructors and destructors must have the same name as the immediately enclosing class. All other members must have names that differ from the name of the immediately enclosing class. If you attempt to prefix a constructor with a type, the compiler will emit an error stating that you cannot define members with the same names as the enclosing type. A constructor initializer of the form base(.) enables the current class's base class constructor— that is, the specific constructor implied by the form of the constructor called—to be called. An initializer taking the form this(.) enables the current class to call another constructor defined within itself. This is useful when you have overloaded multiple constructors and want to make sure that a default constructor is always called. Constructors are not inherited. It's advisable to write a default constructor for every class you write. Constructors are usually public, but can also be private or protected. Static constructors implement the actions required to initialize a class i.e. the constructor is called when the class is loaded. Static constructors are not inherited. Static constructors are invoked automatically, and cannot be invoked explicitly. The exact timing and ordering of static constructor execution is not defined, though several guarantees are provided: 1. The static constructor for a class is executed before any instance of the class is created. 2. The static constructor for a class is executed before any static member of the class is referenced. 3. The static constructor for a class is executed before the static constructor of any of its derived classes are executed. 4. The static constructor for a class never executes more than once. 5. The static constructor cannot have parameters. 6. There's no such thing as a static destructor.
• • •
• • • • • •
Copyright Satish Talim 2001-2002, Study Notes. All Rights Reserved.
Chapter 4 •
Class and Object
Destructors are invoked automatically, and cannot be invoked explicitly. The name of a class destructor is the class name preceded by a tilde (~). They are always public and have no return value. They do not take any arguments, so there can only ever be one for a class. A derived class can hide inherited members by declaring new members with the same name or signature. Note however that hiding an inherited member does not remove the member—it merely makes the member inaccessible in the derived class. The base keyword is in many ways the equivalent of Java's super keyword. Use the new modifier to explicitly hide a member inherited from a base class. A derived class can be used in place of its base class. The as keyword can be used instead of a cast. The advantage over a cast is that if the cast is invalid, you don't have to worry about an exception being thrown. What will happen instead is that the result will be null. When a class-member-declaration does not include any access modifiers, the declaration defaults to private declared accessibility. The access modifiers are associated with members or types (classes etc.) and are not allowed on namespaces. A virtual method is one where the decision on exactly which method to call is delayed until run-time, allowing the dynamic type of the reference to be used. You declare a method as virtual by using the virtual modifier in the base class. When you override a virtual method in a derived class, you can use the override override keyword to signal that you are overriding a virtual method. Remember that the virtual keyword must be used on the base class's method, and the override keyword is used on the derived class's implementation of the method. The interface keyword declares a reference type that has abstract members. Interfaces can contain methods, properties, indexers, and events as members. They can't contain constants, fields (private data members), constructors and destructors or any type of static member. All the members of an interface are public by definition, and the compiler will give you an error if you try to specify any other modifiers on interface members. The static and public modifiers are not permitted on interface methods. Structs are value types rather than reference types, and inheritance is not supported for structs. An enum type declaration defines a type name for a related group of symbolic constants (named integer constants). Each of the named constants has a value, which by default starts at zero and increases by one for each succeeding member. You can give explicit values to any or all of the constants; any that you don't specify get a value one more than the proceeding constant. The default type of the constants is int. The difference between using get/set methods and properties is that to a user, using a property looks like they are getting direct access to the data, whereas in fact the compiler is mapping the call onto the get/set methods. In a property set accessor, the new value for the property is given in an implicit value parameter (the value represents the value passed in from the user). You can omit either the set or get clause in case of a property.
• • • •
• • •
• • •
• •
• •
Copyright Satish Talim 2001-2002, Study Notes. All Rights Reserved.
5. Using .NET Base Classes
Microsoft has supplied a large number of base classes as part of the .NET framework. The base classes are accessible from C#, VB, C++ or any other .NET compliant language.
5.1 The WinCV Tool
WinCV is a class viewer tool, which can be used to examine the classes available in shared assemblies – including all the base classes. For each class, it lists the methods, properties, events and fields using a C# like syntax. At the command prompt type wincv. You then type in a search string that is contained in the class you are looking for in the Searching For box. WinCv will then display a list of classes that contain the string in question in their names. Scan through the names to find the one that looks most suitable. The definition of the selected class is then displayed in the right hand pane. The left hand pane also indicates the namespace the class is contained in – you'll need this in order to be able to use that class in your C# code. The right hand pane of WinCV tells us which assembly the class is defined in. This information is useful to indicate if we need to link in any assemblies when we compile, in order to have access to that class.
5.2 StringBuilder class
This class is used to represent mutable strings. It is convenient for situations in which it is desirable to modify a string, perhaps by removing, replacing, or inserting characters, without creating a new string subsequent to each modification. It is used in conjunction with the String class to carry out modifications upon strings. The methods contained within this class do not return a new StringBuilder object unless specified otherwise. This class is defined in the namespace System.Text. It starts at a predefined size (16 characters by default) and grows dynamically as more string data is added. using System; using System.Text; public class SBTest { public string SqrtIntFaster(int i) { StringBuilder buf = new StringBuilder(50); buf.Append("sqrt(").Append(i).Append(')'); buf.Append(" = ").Append(Math.Sqrt(i)); return buf.ToString(); } static void Main() { SBTest sb = new SBTest(); Console.WriteLine(sb.SqrtIntFaster(25)); } } In the above program SBTest.cs, the StringBuilder constructor has the suggested starting size of 50. This size can increase as more characters are added. The Append() method appends a typed object to the end of the current StringBuilder. The ToString() method is overloaded and it converts a StringBuilder to a String. We have also used Sqrt() method of the Math class.
Copyright Satish Talim 2001-2002, Study Notes. All Rights Reserved.
5...WriteLine("Is Directory... Console.IsFile... Study Notes.." + f.. using System.2 Listing Files in a Folder To explore the contents of a folder...WriteLine("Full Path.. The example FIF....GetFiles().. Console..." + f....WriteLine("Last Write Time... using System... Directory[] childfolders = d1..... Console..ToString())....Chapter 5 Using ." + f.. also in the System...IsFile. Console." + d1. Console. using System...IO namespace.Name).LastWriteTime. } } In general you can use a File object to connect to either a file or a folder." + f.. Console..ToString()).. Console. Note that the .WriteLine("Size in bytes.WriteLine("Full Path.IsDirectory.cs connects to the folder H:\pune-csharp and separately lists the files and folders in it. 91 ..NET base classes include a number of classes that provide a rich set of functionality to access the file system.Length). Console.....WriteLine(childfile..... Console.ToString())." + f. although if you connect to a folder then attempting to access those properties that don't make sense for a folder (such as Length or LastWriteTime) will raise an exception.. public class FIF { public static void Main(String[] args) { Directory d1 = new Directory(@"H:\pune-csharp")." + f.. using some of the properties of the File class....." + d1. 5. Console...Name).WriteLine("Is Directory.. we need another base class – the Directory class. Console.1 Finding out information about a File The program IBF..WriteLine("Connected to folder. public class IBF { public static void Main(String[] args) { File f = new File(@"C:\WINDOWS\BOOTLOG.... } } Copyright Satish Talim 2001-2002....WriteLine("In Folder.. All Rights Reserved... foreach (Directory childfolder in childfolders) Console.Directory).IsDirectory..TXT"). foreach (File childfile in childfiles) Console. These classes are contained in the System.cs will demonstrate how to get information about a file.GetDirectories()..WriteLine("Subfolders contained in this folder:")." + d1....Name).... to read and write to files...." + d1.WriteLine("Is File.WriteLine(childfolder.ToString()). Console.WriteLine("Is File... to move or copy files around..Name)...WriteLine("Connected to file. File[] childfiles = d1. using System.IO.FullName).3 File and Folder operations The ..FullName)..IO namespace.WriteLine("Files contained in this folder:").ToString()).NET base classes generally refer to folders as directories in class and method names. Console...3.3. or to explore folders to check what files are there.IO." + f.NET Base Classes 5.
we bind to one of the files in the H:\pune-csharp folder.CreateSubdirectory("Talim"). subdirectories. sr. public class RTF { public static void Main(String[] args) { File f1 = new File(@"H:\CSHARPTEMP\Hello. // continue reading until end of file string sLine. public class CFF { public static void Main(String[] args) { Directory d1 = new Directory(@"H:\pune-csharp").IO. public class RC { public static void Main(String[] args) { File f1 = new File(@"H:\pune-csharp\Satish").cs: using System. public class DFF { public static void Main(String[] args) { File f1 = new File(@"H:\pune-csharp\Satish"). To read text files we use the class StreamReader.Stream.CONSOLE APPLICATIONS 5.cs: using System. which can represent any stream. d1.ReadLine()) != null) Console. as in example CFF. Study Notes. f1. as in example RC. StreamReader sr = f1. using System. 5. using System.3. d1.IO. and we create both a new empty file and a new empty subfolder there. The example RTF.cs: using System.Delete().C# . } } Next. rename it and copy it. . d1.Close().IO. otherwise false.Delete(true).4 Reading Text Files The available classes are all derived from the class System. } } Here Delete(true) is used to remove directories. f1.cs demonstrates this. Directory d1 = new Directory(@"H:\pune-csharp\Talim"). as in the example DFF.CopyTo(@"H:\pune-csharp\Talim\NewSatish"). using System.3.cs").IO. and contents. } } Next we delete the file NewSatish and the folder Talim.WriteLine(sLine). All Rights Reserved. } 92 Copyright Satish Talim 2001-2002.CreateFile("Satish").3 Copying and Deleting Files We start off by binding to the H:\pune-csharp folder. using System.IO.OpenText(). while ((sLine = sr. using System.
WriteByte(bNext). each time displaying the numeric value of the byte read. 5.txt". public class WBFFS { public static void Main(String[] args) { byte[] bytes = {70. using System. In either case. 73. sw. public class RBFS { public static void Main(String[] args) { File f1 = new File(@"H:\CSHARPTEMP\Hello.WriteLine(6). 10}.Chapter 5 Using . int iNext. 74. FileAccess. } } The StreamWriter constructor takes two parameters: the full name of the file and a boolean that indicates whether data should be appended to the file.Close().cs demonstrates how to use the Stream class to read data. } } Copyright Satish Talim 2001-2002. 72. 75. while ((iNext = s. using System. sw.3.WriteLine(iNext. All Rights Reserved. The program RBFS.ReadByte()) != -1) Console. s.cs"). sw. sw.Close(). Stream s = f1. a byte at a time.3.34). false). the file will be opened if it already exists or created if it does not. } } 5.WriteLine(65. FileStream fs = new FileStream(@"H:\CSHARPTEMP\WBFFS.6 Reading Binary Files Here we use either the Stream or FileStream class.cs explains this: using System. Study Notes..Close(). It opens a file and reads it. 71. The example WTF.Write).. foreach (byte bNext in bytes) fs. FileMode.IO.OpenRead(). If this is false then the contents of the file will be overwritten by the StreamWriter. sw.WriteLine(true).NET Base Classes } 5. using System.5 Writing Text Files For writing text files we use the StreamWriter class.IO. using System.WriteLine("How do you find the C# Workshop?").cs writes out a short text file that contains the letters FGHIJK followed by a carriage return-line feed combination. sw. fs.7 Writing Binary Files The program WBFFS.cs". using System.ToString()). 13.3.WriteLine("We now write some numbers and bool.IO.OpenOrCreate."). public class WTF { public static void Main(String[] args) { StreamWriter sw = new StreamWriter(@"H:\CSHARPTEMP\TEMP. 93 .
in this case turning off the HTTP Keep-alive behavior and getting the protocol version number from the Web server: HttpWebRequest HttpWReq = (HttpWebRequest)WebRequestFactory. ReadWrite and Write.CONSOLE APPLICATIONS The constructor we use takes three parameters: the full pathname of the file.GetResponse(). Your application can automatically follow HTTP redirects by setting the AllowAutoRedirect property true. 5. Custom headers and headers not provided through properties can be accessed by storing them in the Headers property. your application must be prepared to handle redirects as an HTTP protocol error. When AllowAutoRedirect is false. authentication. // Turn off connection keep-alives HttpWReq.ToString().Version.KeepAlive = false. Study Notes. The following sample shows how to access HTTP specific properties. HttpWebRequest is the default class used by WebRequestFactory and does not need to be registered before passing an HTTP Uniform Resource Identifier (URI) to the WebRequestFactory.1 HTTP The HTTP protocol accounts for a large share of all traffic on the Internet. and provide access to common HTTP headers through properties. the WebRequest and WebResponse classes will provide all that is necessary to make the request. The implementation of this architecture in the BCL also includes HTTP-specific extensions to make interacting with web servers easy. In most cases. Should the application require lower-level access to the network. types exist to support TCP and UDP. pre-authentication. The mode and access are enumerated values respectively taken from two further classes in the System. connection management. in situations where direct transport-level access is required. Open. chunking.Create("http://www. proxy support. OpenOrCreate and Truncate. the ResponseURI property of the HttpWebResponse will contain the actual Web resource that responded to the request. HttpWebResponse HttpWResp = (HttpWebResponse)HttpWReq. For the access.C# . Create. there are types that provide raw socket access. The HttpWebRequest and HttpWebResponse classes encapsulate a standard HTTP request and response transaction. 94 Copyright Satish Talim 2001-2002. 5. and the . Finally.4 Networking High level access is performed using a set of types that implement a generic request/response architecture that is extensible to support new protocols. the request or response can be typecast to HttpWebRequest or HttpWebResponse. and HTTP extensions.NET frameworks provide robust support for the HTTP protocol with the HttpWebRequest and HttpWebResponse classes. The possible values for mode is Append. // Look at the HTTP protocol version number returned by the server String ver = HttpWResp. server certificate validation.Create method. CreateNew.pune-csharp. encryption.com"). including pipelining. the mode we are using to open it and the access required.IO namespace: FileMode and FileAccess.1 protocol features. All Rights Reserved. These classes also support most of the HTTP 1. but when access to HTTP-specific features is required. they are Read. These classes are the WebRequest and WebResponse derived classes returned whenever a URI beginning with "http" or "https" is presented to the Create method on the WebRequestFactory. .4. When the request is redirected.
the URI constructor: • • • • Converts the URI scheme to lower case.Chapter 5 Using . The ReadToEnd method of the StreamReader class reads the stream from the current Copyright Satish Talim 2001-2002. The following example creates an instance of the URI class (derived from Object Object). This class is abstract) that reads characters from a byte stream in a particular encoding. The URI class can be transformed from an escaped URI reference to a readable URI reference with the ToString method. To put the URI in canonical form. The WebResponse class is an abstract base class (derived from Object) from which protocolspecific classes are derived. follows the factory design pattern.com/"). relative URIs must be expanded with respect to a base URI so that they are absolute. comparing.2 Generic Request/Response Architecture This is based on Uniform Resource Indicator (URI) and stream I/O.4. The StreamReader class implements a TextReader (represents a reader that can read a sequential stream of characters. The URI class defines the properties and methods for handling URIs. The following example creates a WebRequest instance for an HTTP request. when overridden in a derived class. Client applications should never create WebResponse objects directly. when overridden in a derived class. The character encoding is set by Encoding class and the default buffer size is used. Removes default and empty port numbers. 95 . and combining.ProtocolError. Study Notes. including parsing. The specific class of object returned is based on the URI scheme passed to the Create method.NET Base Classes Applications receive HTTP protocol errors by catching a WebException with the Status set to WebStatus. The GetResponse method in this class. Converts the host name to lower case. The GetResponseStream method in this class. returns the response to an Internet request. The WebRequestFactory class (derived from Object) is a static class that returns an instance of an object derived from WebRequest. The Stream class provides a way to write and read bytes to and from a backing store. The WebRequest is an abstract class derived from Object. Since the URI indicates an HTTP request. which can further be used to create a WebRequest: URI siteURI = new URI("http://www. returns the Stream object used for reading data from the resource referenced in the WebRequest object. 5. This class is abstract. The URI is stored as a canonical URI in "escaped" format. Simplyfies the URI by removing superfluous segments such as "/" and "/test" segments.Create(siteURI). and makes good use of abstract types and interfaces. with all characters with an ASCII value greater than 127 replaced with a hexidecimal representation.pune-csharp. Applications should never create WebRequest objects directly. The URI class stores only absolute URIs. The WebResponse class can be used to access any resource on the network that is addressable with a URI. The Response property contains the WebResponse sent by the server and it can be examined to find the actual HTTP error encountered. the actual instance returned is an instance of HttpWebRequest WebRequest wr = WebRequestFactory. A uniform resource locator is a compact representation of a resource available to your application via the Internet. All Rights Reserved.
WebResponse res = req.IO. All Rights Reserved.GetResponse().C# .exe <http-url> to retrieve a web page using System.ASCII).dll // Run Snarf. Encoding. using System. StreamReader sr = new StreamReader(s. sr.CONSOLE APPLICATIONS position to the end of the stream. Study Notes. Stream s = res.Text. Console.GetResponseStream(). using System. WebRequest req = WebRequestFactory. string doc = sr. class Snarf { public static void Main(string[] args) { URI siteURI = new URI(args[0]).Net. // Snarf.Net.Close().cs // Compile with /r:System. The example Snarf. using System. } } 96 Copyright Satish Talim 2001-2002.ReadToEnd().WriteLine(doc). .Create(siteURI). The Close method closes the StreamReader and releases any system resources associated with the reader.cs below uses the WebRequest and WebResponse classes to retrieve the contents of a URI and display them to the console.
This action might not be possible to undo. Are you sure you want to continue?
We've moved you to where you read on your other device.
Get the full title to continue listening from where you left off, or restart the preview.