C#如何发布符合 .NET Framework 准则的事件

最近项目需要，需要用到.NET框架，C#，以前没有接触过，勉强学学，感觉还行，学习起来不是很难。

最近学校“事件”，其中经典的就是pub-sub模型，在.NET中，有一种方法叫做委托，关键字delegate，这种方式有点类似于接口，但是比借口更加灵活，因为delegate支持广播效果，重载了很多操作父，比如+=,+,-=等，操作起来非常方便。

在pub中，声明一个delegate handler，定义好签名（也就是参数设置），在sub中，我们把pub作为参数传递给sub，并且sub自己主动向pub的handler注册自己的方法（这里可以理解为C中的回掉函数），这样pub需要广播时，直接调用handler就行，而不需要关注是谁注册了handler。

其中C#中EventHandler就是一个delegate，在pub创建一个EventHandler属性，sub中给这个属性赋值就行。

代码如下：

namespace DotNetEvents{    using System;    using System.Collections.Generic;    // Define a class to hold custom event info    public class CustomEventArgs : EventArgs    {        public CustomEventArgs(string s)        {            message = s;        }        private string message;        public string Message        {            get { return message; }            set { message = value; }        }    }    // Class that publishes an event    class Publisher    {        // Declare the event using EventHandler<T>        public event EventHandler<CustomEventArgs> RaiseCustomEvent;        public void DoSomething()        {            // Write some code that does something useful here            // then raise the event. You can also raise an event            // before you execute a block of code.            OnRaiseCustomEvent(new CustomEventArgs("Did something"));        }        // Wrap event invocations inside a protected virtual method        // to allow derived classes to override the event invocation behavior        protected virtual void OnRaiseCustomEvent(CustomEventArgs e)        {            // Make a temporary copy of the event to avoid possibility of            // a race condition if the last subscriber unsubscribes            // immediately after the null check and before the event is raised.            EventHandler<CustomEventArgs> handler = RaiseCustomEvent;            // Event will be null if there are no subscribers            if (handler != null)            {                // Format the string to send inside the CustomEventArgs parameter                e.Message += String.Format(" at {0}", DateTime.Now.ToString());                // Use the () operator to raise the event.                handler(this, e);            }        }    }    //Class that subscribes to an event    class Subscriber    {        private string id;        public Subscriber(string ID, Publisher pub)        {            id = ID;            // Subscribe to the event using C# 2.0 syntax            pub.RaiseCustomEvent += HandleCustomEvent;        }        // Define what actions to take when the event is raised.        void HandleCustomEvent(object sender, CustomEventArgs e)        {            Console.WriteLine(id + " received this message: {0}", e.Message);        }    }    class Program    {        static void Main(string[] args)        {            Publisher pub = new Publisher();            Subscriber sub1 = new Subscriber("sub1", pub);            Subscriber sub2 = new Subscriber("sub2", pub);            // Call the method that raises the event.            pub.DoSomething();            // Keep the console window open            Console.WriteLine("Press Enter to close this window.");            Console.ReadLine();        }    }}