为什么调用前处理函数分配给一个事件？ [英] Why assign a handler to an event before calling it?

问题描述

基本上，我已经看到了这个使用的所有经常：

Basically, I've seen this used all to often:

    public event MyEventHandler MyEvent;

    private void SomeFunction()
    {
        MyEventHandler handler = this.MyEvent;

        if (handler != null)
        {
            handler(this, new MyEventArgs());
        }
    }

当它可以很容易地像这样来完成：

When it could just as easily be done like so:

    public event MyEventHandler MyEvent;

    private void SomeFunction()
    {
        if (MyEvent != null)
        {
            MyEvent(this, new MyEventArgs());
        }
    }

所以，我失去的东西吗？是否有某种原因，人们将事件指派给一个处理程序，然后抬起处理器，而不是事件本身？难道仅仅是最佳实践？

So, am I missing something? Is there some reason people assign the event to a handler, then raise the handler instead of the event itself? Is it just "best practice"?

推荐答案

分配给本地变量保证，如果事件被在<$之间注销C $ C>如果和实际调用，调用列表将不为空（因为变量都会有一个的复制的原调用列表）。

The assignment to a local variable ensures that if the event gets unregistered between the if and the actual invocation, the invocation list will not be null (since the variable will have a copy of the original invocation list).

这可以很容易地发生在多线程代码，其中间检查空和发射可能被另一个线程注销该事件。

This can easily happen in multithreaded code, where between checking for a null and firing the event it may be unregistered by another thread.

请参阅这个，以便问题和答案。

See this SO question and answers.

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