Why assign a handler to an event before calling it?

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"?

score 13 · Accepted Answer · edited May 23 '17 at 12:09

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.

score 2 · Answer 2 · answered Feb 28 '11 at 20:50

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?

answered Feb 28 '11 at 20:50

Marty

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Why assign a handler to an event before calling it?

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