title: Add method to a class from another project
body: <p>Is it possible to add a method to a class from another project?</p>
  <p>I have a class:</p>
  <pre><code>namespace ProjectLibrary
  {
      public class ClassA
      {
          // some methods
      }
  }
  </code></pre>
  <p>I want to add a method to save my object in a file, but I don't want to add this method in my project <code>ProjectLibrary</code> because I don't want to add reference to System.IO (I want to use this project for android application and PC application).</p>
  <p>Is there a possibility to add a method <code>SaveToFile()</code> usable only in another project? Or create an abstract method in ClassA but define it in other project.</p>
  <p>Like this :</p>
  <pre><code>using ProjectLibrary;
  namespace ProjectOnPC
  {
      void methodExample()
      {
          ClassA obj = new ClassA();
          // do something
          obj.SaveToFile();// =&gt; available only in namespace ProjectOnPC
      }
  }
  </code></pre>
  <p>Thank you for help</p>