title: Derive abstract class from non-abstract class
body: <p>Is it OK to derive an abstract class from a non-abstract class or is there something wrong with this approach?</p>
  
  <p>Here´s a little example:</p>
  
  <pre><code>public class Task {
    // Some Members
  }
  
  public abstract class PeriodicalTask : Task {
    // Represents a base class for task that has to be done periodicaly.
    // Some additional Members
  }
  
  public class DailyTask : PeriodicalTask {
    // Represents a Task that has to be done daily.
    // Some additional Members
  }
  
  public class WeeklyTask : PeriodicalTask {
    // Represents a Task that has to be done weekly.
    // Some additional Members
  }
  </code></pre>
  
  <p>In the example above I do not want to make the class Task abstract, because I want to instantiate it directly. PeriodicalTask should inherit the functionality from Task and add some additional members but I do not want to instantiate it directly. Only derived class of PeriodicalTask should be instantiated.</p>