实现事件只能注册一次，自定义事件访问器

直接上代码了，很简单的，一看就OK

using System;
using System.Collections.Generic;
using System.Text;

namespace DelegateTest
{
    class Program
    {

        static void Main(string[] args)
        {
            Pig p = new Pig();
            p.Single += new Pig.SingleDel(p_Single1);
            p.Single += new Pig.SingleDel(p_Single2);
            p.Mulit += new Pig.MulitDel(p_Mulit1);
            p.Mulit += new Pig.MulitDel(p_Mulit2);

            p.TriggerSingle();
            p.TriggerMulit();

            Console.Read();
        }

        static void p_Mulit1()
        {
            Console.WriteLine( "第一次注册Mulit ");
        }
        static void p_Mulit2()
        {
           Console.WriteLine( "第二次注册Mulit ");
        }
        static void p_Single1()
        {
            Console.WriteLine("第一次注册Single ");
        }
        static void p_Single2()
        {
            Console.WriteLine("第二次注册Single ");
        }

    }

    public class Pig
    {
        public delegate void SingleDel(); //单播，只能被订阅一次(自定义访问器来实现)修改add里面的逻辑可以实现其它注册方式

        public delegate void MulitDel();//多播，可以被订阅多次

        public Pig()
        {

        }
        private event SingleDel single;
        public event SingleDel Single
        {
            add
            {
                if (single != null)
                {
                    Delegate[] dels = single.GetInvocationList();
                    if (dels.Length > 0)
                    {
                        foreach (Delegate i in dels)
                        {
                            single = Delegate.Remove(single, i) as SingleDel;
                        }
                    }
                    single = Delegate.Combine(single, value) as SingleDel;
                }
                else
                    single += value;
            }
            remove
            {
                if (single == null)
                    return;
                single -= value;
            }
        }
        public event MulitDel Mulit;

        public void TriggerSingle()
        {
            if (single != null)
                single();

        }
        public void TriggerMulit()
        {
            if (Mulit != null)
                Mulit();
        }
    }
}