C#事件具体实现步骤

事件记录簿v1.0.1

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定义一个类型来监听事件

   public sealed class Fax
    {
        public Fax(MailManager mm)
        {
            // 注册回调方法FaxMsg
            mm.NewMail += FaxMsg;
        }

        // 当NewEmail发生时，触发这个方法执行
        private void FaxMsg(Object sender, NewMailEventArgs e)
        {
            // 'sender'用来识别是MailManager对象
            // 'e' MailManager要传递的额外的事件信息
            Console.WriteLine("Faxing mail message:");
            Console.WriteLine("   From={0}, To={1}, Subject={2}",
               e.From, e.To, e.Subject);
        }

        //从事件从注销方法，以后不在收到通知
        public void Unregister(MailManager mm)
        {
            mm.NewMail -= FaxMsg;
        }
    }

C#内置对事件的支持，编译会根据+=生成代码：mm.add_NewMail(new EventHandler<NewMailEventArgs>(this.FaxMsg));

编译器构造了一个EventHandler<NewMailEventsArgs>委托对象来封装Fax类的FaxMsg方法。然后，C#编译器调用MailManager的add_NewMail方法，将它传递到新的委托。IL如下：

    .event class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs> NewMail
    {
        .addon instance void TypeTest.MailManager::add_NewMail(class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs>)
        {
            .locals init (
                [0] class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs> V_0,
                [1] class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs> V_1,
                [2] class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs> V_2,
                [3] bool V_3
            )

            IL_0000: ldarg.0
            IL_0001: ldfld class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs> TypeTest.MailManager::NewMail
            IL_0006: stloc.0
            .loop
            {
                IL_0007: ldloc.0
                IL_0008: stloc.1
                IL_0009: ldloc.1
                IL_000a: ldarg.1
                IL_000b: call class [mscorlib]System.Delegate [mscorlib]System.Delegate::Combine(class [mscorlib]System.Delegate,  class [mscorlib]System.Delegate)
                IL_0010: castclass class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs>
                IL_0015: stloc.2
                IL_0016: ldarg.0
                IL_0017: ldflda class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs> TypeTest.MailManager::NewMail
                IL_001c: ldloc.2
                IL_001d: ldloc.1
                IL_001e: call class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs> [mscorlib]System.Threading.Interlocked::CompareExchange<class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs>>(!!0&,  !!0,  !!0)
                IL_0023: stloc.0
                IL_0024: ldloc.0
                IL_0025: ldloc.1
                IL_0026: ceq
                IL_0028: ldc.i4.0
                IL_0029: ceq
                IL_002b: stloc.3
                IL_002c: ldloc.3
                IL_002d: brtrue.s IL_0007
            }
            IL_002f: ret
        }
        .removeon instance void TypeTest.MailManager::remove_NewMail(class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs>)
        {
            .locals init (
                [0] class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs> V_0,
                [1] class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs> V_1,
                [2] class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs> V_2,
                [3] bool V_3
            )

            IL_0000: ldarg.0
            IL_0001: ldfld class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs> TypeTest.MailManager::NewMail
            IL_0006: stloc.0
            .loop
            {
                IL_0007: ldloc.0
                IL_0008: stloc.1
                IL_0009: ldloc.1
                IL_000a: ldarg.1
                IL_000b: call class [mscorlib]System.Delegate [mscorlib]System.Delegate::Remove(class [mscorlib]System.Delegate,  class [mscorlib]System.Delegate)
                IL_0010: castclass class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs>
                IL_0015: stloc.2
                IL_0016: ldarg.0
                IL_0017: ldflda class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs> TypeTest.MailManager::NewMail
                IL_001c: ldloc.2
                IL_001d: ldloc.1
                IL_001e: call class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs> [mscorlib]System.Threading.Interlocked::CompareExchange<class [mscorlib]System.EventHandler`1<class TypeTest.NewMailEventArgs>>(!!0&,  !!0,  !!0)
                IL_0023: stloc.0
                IL_0024: ldloc.0
                IL_0025: ldloc.1
                IL_0026: ceq
                IL_0028: ldc.i4.0
                IL_0029: ceq
                IL_002b: stloc.3
                IL_002c: ldloc.3
                IL_002d: brtrue.s IL_0007
            }
            IL_002f: ret
        }
    }

当MailManager对象触发了事件，Fax对象的FaxMsg方法会被调用，这个方法的一个引用会被作为MailManager对象的第一个参数sender传递给MailManager。多数时候该参数被忽略。当Fax对象想访问MailManager对象来响应事件通知时会使用。第二个参数是NewMailEventArgs对象，传递额外的信息。

当一个对象不再希望接收某事件的通知时，应该从事件里面注销。只要某一个对象的任何一个方法还在事件里面，这个对象就不会被GC回收。如果类型实现了IDisposable的Dispose方法，该实现应该注销所有在事件里面的方法。