C# 中的多播委托奇怪行为?

问题描述

我有一个简单的事件:

public class ClassA
{
    public event Func<string, int> Ev;
    public int Do(string l)
    {
        return Ev(l);
    }
}

还有两种方法:

  static int Display(string k)
        {
            return k.Length;
        }

  static int Display_2(string k)
        {
            return k.Length*10;
        }

我正在注册这个活动:

 ClassA a = new ClassA();
 a.Ev += Display;
 a.Ev += Display_2;

现在，我正在执行:

   Console.WriteLine(a.Do("aaa")); 

输出:

什么???

• 他在调用列表中有 2 个方法！它确实运行了它们，但为什么它只显示last注册的结果?

• he has in invocation list 2 methods ! it did run them , but why does it shows only the result from the last registration ?

3"的结果到哪里去了?(第一次调用)?(虽然 display+display_2 都被执行了......我没想到 console.write 会遍历结果.但也没想到他会决定展示哪个.)

Where does the result of "3" has gone ? ( the first invocation ) ? ( although both display+display_2 was executed... I didn't expect console.write to iterate through results . but also didn't expect him to decide which to show.)

推荐答案

这里涉及三个方面:

1. 活动的实施
2. 委托组合的行为
3. 调用其调用列表中有多个条目的委托的行为

对于第 1 点，您有一个 field-like 事件.C# 4 规范的第 10.8.1 节给出了一个示例，并指出:

For point 1, you have a field-like event. Section 10.8.1 of the C# 4 spec gives an example, and states that:

在 Button 类的声明之外，Click 成员只能在 += 的左侧使用，并且-= 运算符，如

Outside the declaration of the Button class, the Click member can be used only on the left-hand saide of the += and -= operators, as in

b.Click += new EventHandler(...);

将委托附加到 Click 事件的调用列表

which appends a delegate to the invocation list of the Click event

(强调我的).该规范还明确指出，类字段事件会创建一个委托字段，该字段用于在类内进行调用.

(emphasis mine). The spec also makes it clear that a field-like event creates a delegate field, which is used from within the class for invocation.

更一般地说(第 2 点)，C# 4 规范的第 7.8.4 节讨论了通过 + 和 += 进行委托组合:

More generally (point 2), section 7.8.4 of the C# 4 spec talks about delegate combination via + and +=:

委托组合.每个委托类型都隐式提供以下预定义运算符，其中 D 是委托类型:

Delegate combination. Every delegate type implicitly provides the following predefined operator, where D is the delegate type:

D operator +(D x, D y)

二进制+ 操作符在两个操作数都属于某个委托类型D 时执行委托组合.[...跳过 x 或 y 为空的位 ...] 否则，操作的结果是一个新的委托，当被调用时，调用第一个操作数，然后调用第二个操作数.

The binary + operato performs delegate combination when both operands are of some delegate type D. [... skip bits where x or y are null ...] Otherwise, the result of the operation is a new delegate that, when invoked, invokes the first operand and then invokes the second operand.

(再次强调我的.)

最后，第 3 点 - 事件调用和返回值.C# 规范第 15.4 节指出:

Finally, point 3 - event invocation and return values. Section 15.4 of the C# spec states:

如果委托调用包含输出参数或返回值，则它们的最终值将来自列表中最后一个委托的调用.

If the delegate invocation includes output parameters or a return value, their final value will come from the invocation of the last delegate in the list.

更一般地说，它取决于事件的实现.如果您使用使用正常"的事件实现.委托组合/移除步骤，一切有保障.如果你开始编写一个做疯狂事情的自定义实现，那就是另一回事了.

More generally, it depends on the event implementation. If you use an event implementation which uses the "normal" delegate combination/removal steps, everything is guaranteed. If you start writing a custom implementation which does crazy things, that's a different matter.

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