How do I wait for an asynchronous event to be called?

Question

I have a method in C# that requires waiting for an event to occur. It's sending data to a client, but it has to wait for an event Action to be fired (which is fired when a response packet is received). How to I block execution of the method until the handler is called?

score 2 · Answer 1 · answered Jun 09 '16 at 19:20

2

This solution worked nicely, and removes the handler from the event once it is no longer needed.

AutoResetEvent evt = new AutoResetEvent(false);
Action handler = () => {
  //Do stuff
  evt.Set();
}

Event += handler;
evt.WaitOne();
Event -= handler;

However, there is one major caveat (with a fairly easy workaround): The calling thread will block indefinitely until the event is called. This means that if the event is never called, your program will hang. The fix for this is to simply change evt.WaitOne(); to evt.WaitOne(SOME_TIMEOUT);

answered Jun 09 '16 at 19:20

rookie1024

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7
18

If you're just waiting for one anyway, why not use ManualResetEventSlim? You don't really need it to reset, and Slim gives slightly better performance if you expect the response quickly. – Tim Copenhaver Jun 09 '16 at 19:30
Good question. Probably should. – rookie1024 Jun 09 '16 at 19:51

score 1 · Accepted Answer · edited May 23 '17 at 12:31

An alternative would be to use a TaskCompletionSource See https://social.msdn.microsoft.com/Forums/en-US/a30ff364-3435-4026-9f12-9bd9776c9a01/signalling-await?forum=async

It then becomes:

TaskCompletionSource tsc = new TaskCompletionSource<bool>();
Action handler = () => {
  //Do stuff
  tsc.SetResult(true);
}

Event += handler;
await tsc.Task;
Event -= handler;

The referenced discussion explains why this is a more lightweight solution.

Also, it won't block the thread and allows for a timeout (in case the event is not called) and/or cancellation (in case the application is closed down) by using a CancellationTokenSource, see Timeout an async method implemented with TaskCompletionSource

As it happens, I based my solution off of this - I just thought it would be faster to use wait handles. — rookie1024, Jun 10 '16 at 00:56

How do I wait for an asynchronous event to be called?

2 Answers2