public delegate void OperationOnDelegate(int x,int y);
 internal class Program
 {
     public static void Add(int x,int y)
     {
         Console.WriteLine($"Add:{x + y}");

     }

     public static void Sub(int x, int y)
     {
         Console.WriteLine($"Sub:{x - y}");


     }

     public static void Mul(int x, int y)
     {
         Console.WriteLine($"Sub:{x * y}");
     }




     static void Main(string[] args)
     {
         OperationOnDelegate op;
         op = Add;
         op(10, 20);
         op = Sub;
         op(10, 20);
         op = Mul;
         op(10, 20);
         op = Add;
         op += Sub;
         op += Mul;
         op(20, 10);
     }
 }