C#-delegate

using System;using System.Collections.Generic;using System.Linq;using System.Text;namespace delegateDemo{    public delegate int NumberOperate(int x,int y);    public class SimpleMath    {        public static int add(int x, int y)        { return x + y; }        public static int sub(int x, int y)        { return x - y; }    }        class Program    {        static void Main(string[] args)        {            int a = 10, b = 20;            NumberOperate numOp = new NumberOperate(SimpleMath.add);            Console.WriteLine("{0}+{1}={2}",a,b,numOp(a,b));            numOp = new NumberOperate(SimpleMath.sub);            Console.WriteLine("{0}-{1}={2}", a, b, numOp(a, b));            Console.ReadLine();        }    }}