说说Delegate和Event
Delegate和Event,在C#中,常常看到他们在一起使用,这其中有什么奥秘呢,在这里我说说我看到的,我们以下边的代码为例:
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using System;
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//Delegate
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delegate void UpdateDelegate();
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//Subject
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class Subject
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{
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public event UpdateDelegate UpdateHandler;
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// Methods
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public void Attach(UpdateDelegate ud)
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{
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UpdateHandler += ud;
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}
16![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public void Detach(UpdateDelegate ud)
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{
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UpdateHandler -= ud;
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}
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public void Notify()
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{
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if (UpdateHandler != null) UpdateHandler();
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}
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}
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public class Client
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{
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static void UpdateDelegateMethod()
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{
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Console.WriteLine("hello world");
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}
34![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public static void Main(string[] args)
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{
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Subject sj = new Subject();
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sj.UpdateHandler += new UpdateDelegate(UpdateDelegateMethod);
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sj.Attach(new UpdateDelegate(UpdateDelegateMethod));
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sj.Notify();
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}
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}
类Subjuect中的委托(UpdateDelegate)之前有event关键字,它的IL视图为:![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
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![](https://www.cnblogs.com/images/cnblogs_com/fengfeng/EventAndDelegate1.gif)
如果我们把委托(UpdateDelegate)之前的event关键字去掉,它的IL视图为:
![](https://www.cnblogs.com/images/cnblogs_com/fengfeng/Delegate1.gif)
两幅图对比之下,我们就会发现,加上Event关键之和去掉Event关键字所产生的IL代码大不相同。
我们再来看看Main函数中使用的不同
如果我们去掉委托(UpdateDelegate)之前的event关键字,下边Main方法中的写法都是对的,在编译的时候不会报错。
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public static void Main(string[] args)
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{
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Subject sj = new Subject();
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sj.UpdateHandler = UpdateDelegateMethod;
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sj.UpdateHandler = new UpdateDelegate(UpdateDelegateMethod);
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sj.UpdateHandler += new UpdateDelegate(UpdateDelegateMethod);
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sj.Attach(new UpdateDelegate(UpdateDelegateMethod));
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sj.Notify();
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}
如果我们在委托(UpdateDelegate)之前加上event关键字![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
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public static void Main(string[] args)
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{
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Subject sj = new Subject();
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//加上event关键字,下边两段代码要报错
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//事件“Subject.UpdateHandler”只能出现在 += 或 -= 的左边(从类型“Subject”中使用时除外)
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//sj.UpdateHandler = UpdateDelegateMethod;
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//sj.UpdateHandler = new UpdateDelegate(UpdateDelegateMethod);
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sj.UpdateHandler += new UpdateDelegate(UpdateDelegateMethod);
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sj.Attach(new UpdateDelegate(UpdateDelegateMethod));
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sj.Notify();
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}
C#中,delegate是multicast的。multicast就是delegate可以同时指向多个函数。一个multicast delegate维护着一个函数的list,如果我们没有用+=,而是直接把函数指派给了delegate,这样的话,其他已经钩上delegate的函数都被去掉了,从这里我们就可以看出,使用event可以防止我们直接把函数指派给delegate,从机制上保证了delegate函数链不被破坏。
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