Test is and as

   

/*
Learning C# 
by Jesse Liberty

Publisher: O'Reilly 
ISBN: 0596003765
*/
 using System;

 namespace InterfaceDemo
 {
     interface IStorable
     {
         void Read();
         void Write(object obj);
         int Status { get; set; }

     }

     // the Compressible interface is now the
     // base for ILoggedCompressible
     interface ICompressible
     {
         void Compress();
         void Decompress();
     }

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


     // Document implements both interfaces
     class Document : IStorable, ILoggedCompressible
     {
         // the document constructor
         public Document(string s)
         {
             Console.WriteLine("Creating document with: {0}", s);

         }

         // implement IStorable
         public void Read()
         {
             Console.WriteLine(
                 "Implementing the Read Method for IStorable");
         }

         public void Write(object o)
         {
             Console.WriteLine(
                 "Implementing the Write Method for IStorable");
         }

         public int Status
         {
             get { return status; }
             set { status = value; }
         }

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

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

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

         // hold the data for IStorable's Status property
         private int status = 0;
     }


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

           // cast using as, then test for null
           IStorable isDoc = doc as IStorable;
           if (isDoc != null)
           {
               isDoc.Read();
           }
           else
           {
               Console.WriteLine("Could not cast to IStorable");
           }



           ILoggedCompressible ilDoc = doc as ILoggedCompressible;
           if (ilDoc != null)
           {
               Console.Write("
Calling both ICompressible and ");
               Console.WriteLine("ILoggedCompressible methods...");
               ilDoc.Compress();
               ilDoc.LogSavedBytes();
           }
           else
           {
               Console.WriteLine("Could not cast to ILoggedCompressible");
           }

           // cast using as, then test for null
           ICompressible icDoc = doc as ICompressible;
           if (icDoc != null)
           {
               Console.WriteLine(
                   "
Treating the object as Compressible... ");
               icDoc.Compress();
           }
           else
           {
               Console.WriteLine("Could not cast to ICompressible");
           }
       }

       [STAThread]
       static void Main()
       {
          TesterISAS t = new TesterISAS();
          t.Run();
       }
    }
 }