using System;

class EventInt{
   Int32 val;   

   public Int32 Value{
      get{return val;}
      set{
         if(Changed != null)
            Changed(value, val);
         val = value;         
      }
   }

   public event Callback Changed;
   public delegate void Callback(Int32 newVal, Int32 oldVal);
}

class App{
   public static void Main(){
      EventInt num = new  EventInt();

      num.Changed += new EventInt.Callback(HandleChange);

      num.Value = 20;
      num.Value += 10;      
   }

   static void HandleChange(Int32 newVal, Int32 oldVal){
      Console.WriteLine("Changed from {0} to {1}", oldVal, newVal);  
   }
}