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:
- 活动的实施
- 委托组合的行为
- 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
Buttonclass, theClickmember 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
Dis 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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