It is possible to extend an existing interface to add new methods or members. For example, you might extend ICompressible with a new interface, ILoggedCompressible, which extends the original interface with methods to keep track of the bytes saved. One such method might be called LogSavedBytes( ). The following code creates a new interface named ILoggedCompressible that is identical to ICompressible except that it adds the method LogSavedBytes:

    interface ILoggedCompressible : ICompressible
    {
     void LogSavedBytes(  );
    }

Classes are now free to implement either ICompressible or ILoggedCompressible, depending on whether they need the additional functionality. If a class does implement ILoggedCompressible, it must implement all the methods of both ILoggedCompressible and also ICompressible. Objects of that type can be cast either to ILoggedCompressible or to ICompressible.

Example 13-4 extends ICompressible to create ILoggedCompressible, and then casts the Document first to be of type IStorable, then to be of type ILoggedCompressible. Finally, the example casts the Document object to ICompressible. This last cast is safe because any object that implements ILoggedCompressible must also have implemented ICompressible (the former is a superset of the latter). This is the same logic that says you can cast any object of a derived type to an object of a base type (that is, if Student derives from Human, then all Students are Human, even though not all Humans are Students).

using System;

namespace ExtendingInterfaces
{
   interface ICompressible
   {
      void Compress(  );
      void Decompress(  );
   }

   // extend ICompressible to log the bytes saved
   interface ILoggedCompressible : ICompressible
   {
      void LogSavedBytes(  );
   }

   public class Document :  ILoggedCompressible
   {

      public Document( string s )
      {
         Console.WriteLine( "Creating document with: {0}", s );
      }


      #region

      public void Compress(  )
      {
         Console.WriteLine( "Implementing Compress" );
      }

      public void Decompress(  )
      {
         Console.WriteLine( "Implementing Decompress" );
      }


      public void LogSavedBytes(  )
      {
         Console.WriteLine( "Implementing LogSavedBytes" );
      }

      #endregion //ILoggedCompressible

   }

   class Tester
   {
      public void Run(  )
      {
         Document doc = new Document( "Test Document" );

         ILoggedCompressible myLoggedCompressible = doc as ILoggedCompressible;
         if ( myLoggedCompressible != null )
         {
            Console.Write( "
Calling both ICompressible and " );
            Console.WriteLine( "ILoggedCompressible methods..." );
            myLoggedCompressible.Compress(  );
            myLoggedCompressible.LogSavedBytes(  );
         }
         else
         {
            Console.WriteLine( "Something went wrong! Not ILoggedCompressible" );
         }
      }

      static void Main(  )
      {
         Tester t = new Tester(  );
         t.Run(  );
      }
   }
}

The output looks like this:

    Creating document with: Test Document

    Calling both ICompressible and ILoggedCompressible methods...
    Implementing Compress
    Implementing LogSavedBytes

Example 13-4 starts by creating the ILoggedCompressible interface, which extends the ICompressible interface:

    // extend ICompressible to log the bytes saved
    interface ILoggedCompressible : ICompressible
    {
       void LogSavedBytes(  );
    }

Notice that the syntax for extending an interface is the same as that for deriving from a class. This extended interface defines only one new method (LogSavedBytes( )), but any class implementing this interface must also implement the base interface (ICompressible) and all its members. (In this sense, it is reasonable to say that an ILoggedCompressible object is-a ICompressible object.)