Events and multithreading once again

Question

I\'m worried about the correctness of the seemingly-standard pre-C#6 pattern for firing an event:

EventHandler localCopy = SomeEvent;
if (localCopy != null)
             


        

Answer 1

The optimizer is not allowed to transform the pattern of code stored in a local variable that is later used into having all uses of that variable just be the original expression used to initialize it. That's not a valid transformation to make, so it's not an "optimization". The expression can cause, or be dependent on, side effects, so that expression needs to be run, stored somewhere, and then used when specified. It would be an invalid transformation of the runtime to resolve the event to a delegate twice, when your code only has it done once.

As far as re-ordering is concerned; the re-ordering of operation is quite complicated with respect to multiple threads, but the whole point of this pattern is that you're now doing the relevant logic in a single threaded context. The value of the event is stored into a local, and that read can be ordered more or less arbitrarily with respect to any code running in other threads, but the read of that value into the local variable cannot be re-reordered with respect to subsequent operations of that same thread, namely the if check or the invocation of that delegate.

Given that, the pattern does indeed do what it intends to do, which is to take a snapshot of the event and invokes all handlers, if there are any, without throwing a NRE due to there not being any handlers.