I was just reading a page on events on MSDN, and I came across a snippet of example code that is puzzling me.
The code in question is this:
// Make a temporary copy of the event to avoid possibility of
// a race condition if the last subscriber unsubscribes
// immediately after the null check and before the event is raised.
EventHandler<CustomEventArgs> handler = RaiseCustomEvent;
I understand the intentions of the code, but I fail to see how that particular line is making a copy of anything. All it is doing is copying the reference; it's not actually making a deep copy of the delegate instance. So to that end, it doesn't actually prevent the race condition at all.
Am I missing something obvious here?
Delegates are immutable, so the reference obtained in that code is guaranteed to not change. If a user subscribes or unsubscribes after the null check, a new delegate will be created and set to the event. However, since you have a reference to a completely different object and invoke that, you don't have to worry about it being null.