Thread Safe Event Handling

Question

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Checking for null before event dispatching… thread safe?
Raise event thread safely - best practice

protected void NotificationEvent(Object sender, EventArgs e)
{
    // Copy to a temporary variable to be thread-safe
    EventHandler<EventArgs> tmp = mNotification;
    if (tmp!= null)
    {
        tmp(this, null);
    }
}

How does copying mNotification makes it thread-safe. Can someone explain?

and to http://stackoverflow.com/questions/282653/checking-for-null-before-event-dispatching-thread-safe — PVitt, Oct 11 '11 at 12:08
As always, Eric Lippert has the answer: http://blogs.msdn.com/b/ericlippert/archive/2009/04/29/events-and-races.aspx — Ilian, Oct 11 '11 at 12:22

L.B · Accepted Answer · 2011-10-11T12:16:23.407

3

If it were

if (mNotification!=null)
{
    mNotification(this, null);
}

mNotification could be set to null by another thread between if (mNotification!=null) and mNotification(this, null);

edited Oct 11 '11 at 12:16

answered Oct 11 '11 at 12:10

L.B

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score 1 · Answer 2 · answered Oct 11 '11 at 12:12

What it does is to make a copy of the reference to the original event at that particular moment in time, so that if it is subsequently used after the null check, it will point to a reference that does not evaluate to null. If this pattern is not used, then a check can be made if it is not null, all handlers could be unsubscribed on another thread, then the event would be null by the time it was invoked. Copying the original reference removes this potential threading issue.

Thread Safe Event Handling

2 Answers2