Event handler raising method convention
I was just browsing and came across this question:
Action vs delegate event
The answer from nobug included this code:
protected virtual void OnLeave(EmployeeEventArgs e) {
var handler = Leave;
if (handler != null)
handler(this, e);
}
Resharper also generates similar code when using the "create raising method" quick-fix.
My question is, why is this line necessary?:
var handler = Leave;
Why is it b开发者_C百科etter than writing this?:
protected virtual void OnLeave(EmployeeEventArgs e) {
if (Leave != null)
Leave(this, e);
}
It's better because there is a tiny possibility that Leave
becomes null after the null check, but before the invocation (which would cause your code to throw a NullReferenceException
). Since the delegate type is immutable, if you first assign it to a variable this possibility goes away; your local copy will not be affected by any changes to Leave
after the assignment.
Note though that this approach also creates a issue in reverse; it means that there is a (tiny, but existing) possibility that an event handler gets invoked after it has been detached from the event. This scenario should of course be handled gracefully as well.
In a multi-threaded application, you could get a null reference exception if the caller unregisters from the event. The assignment to a local variable protects against this.
Odds are you'd never see this (until it hurts you the worst). Here's a way of looking at it that shows the problem...
protected virtual void OnLeave(EmployeeEventArgs e) {
if (Leave != null) //subscriber is registered to the event
{
//Subscriber unregisters from event....
Leave(this, e); //NullReferenceException!
}
}
Here's an excellent explanation by Eric Lippert :
Events and races
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