为什么在调用之前为事件分配处理程序？

Question

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());
        }
    }

When it could just as easily be done like so:

    public event MyEventHandler MyEvent;

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

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"?

Answer 1

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).

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

See this SO question and answers.

Answer 2

Thread safety.

What happens if between the time you check if MyEvent is null and you fire MyEvent, another thread comes along and unsubscribes from the event?