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        C# 中的多播委托奇怪行为?

        时间:2023-11-11

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                • 本文介绍了C# 中的多播委托奇怪行为?的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着跟版网的小编来一起学习吧!

                  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:

                  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 ++= 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 xy 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.

                  这篇关于C# 中的多播委托奇怪行为?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持跟版网!

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