Use interface as reference : Object Reference « Class « C# / CSharp Tutorial

1. C# / CSharp Tutorial
2. Class
3. Object Reference

using System;

interface Interface1
{
   void PrintOut(string s);
}

class MyClass : Interface1
{
   public void PrintOut(string s)
   {
      Console.WriteLine("Calling through: {0}", s);
   }
}

class MainClass
{
   static void Main()
   {
      MyClass mc = new MyClass();         
      mc.PrintOut("object.");             

      Interface1 ifc = (Interface1)mc;    
      ifc.PrintOut("interface.");         
   }
}

Calling through: object.
Calling through: interface.