复制代表 [英] Copying delegates

问题描述

我刚刚在MSDN上的事件上阅读了一页，我遇到了令人困惑的示例代码片段。

I was just reading a page on events on MSDN, and I came across a snippet of example code that is puzzling me.

有问题的代码是这样的：

The code in question is this:

// Make a temporary copy of the event to avoid possibility of
// a race condition if the last subscriber unsubscribes
// immediately after the null check and before the event is raised.
EventHandler<CustomEventArgs> handler = RaiseCustomEvent;

我了解代码的意图，但是我看不到该特定行是如何复制的任何东西。它所做的只是复制 reference ；它实际上不是在复制委托实例的深层副本。因此，这实际上并没有完全阻止比赛条件。

I understand the intentions of the code, but I fail to see how that particular line is making a copy of anything. All it is doing is copying the reference; it's not actually making a deep copy of the delegate instance. So to that end, it doesn't actually prevent the race condition at all.

我在这里错过了明显的东西吗？

Am I missing something obvious here?

推荐答案

代理是不可变的，因此可以保证在该代码中获得的引用不会更改。如果在进行空检查后用户订阅或取消订阅，将创建一个新的委托并将其设置为事件。但是，由于您引用了完全不同的对象并调用了该对象，因此不必担心它为null。

Delegates are immutable, so the reference obtained in that code is guaranteed to not change. If a user subscribes or unsubscribes after the null check, a new delegate will be created and set to the event. However, since you have a reference to a completely different object and invoke that, you don't have to worry about it being null.

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