using System;

public interface IMyInterface

{

   void DoWork();

}

public class MyClassWithCallback

{

   private IMyInterface _myInterface;

   public void AddCallback(IMyInterface myInterface)

   {

      _myInterface = myInterface;

   }

   public void RemoveCallback()

   {

      _myInterface = null;

   }

   public void DoWork()

   {

      if (_myInterface != null)

          _myInterface.DoWork();

   }

}

public class MyBaseClass : IMyInterface

{

   public void DoWork()

   {

      Console.WriteLine("Call interface method.");

   }

   public static void Main()

   {

      MyClassWithCallback mc = new MyClassWithCallback();

      IMyInterface mi = new MyBaseClass();

      mc.AddCallback(mi);

      mc.DoWork();

      mc.RemoveCallback();

   }

}

​