为什么在调用事件之前为其分配处理程序？ [英] Why assign a handler to an event before calling it?

问题描述

基本上，我已经看到这一切经常使用：

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());
        }
    }

它可以像这样轻松完成：

When it could just as easily be done like so:

    public event MyEventHandler MyEvent;

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

那么，我错过了什么？有没有人将事件分配给处理程序，然后提出处理程序而不是事件本身？只是最佳实践？

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

对局部变量的赋值确保如果事件在如果和实际调用，则调用列表不会为空（因为变量将具有原始调用列表的复制）。

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中，其中检查null并触发可能被另一个线程注销的事件。

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

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