仅在变量为零时执行处置

Question

Rough context: messaging system, Communicator sends messages (increases field _messagesInTransmission ), receives ACK or NAK on another thread (decreases _messagesInTransmission ).

If an instance of Communicator is disposed it should stop its underlying transport instance as soon as _messagesInTransmission hits zero the next time.

How to implement this in a thread-safe way?

public void Dispose()
{
    _state = State.Stopping; // immediately change state in order to prevent additional messages to be sent

    // TODO: wait for _messagesInTransmission to become zero
}

Does the .NET framework support me with this task?

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A simple solution would be a while(_mIT != 0) loop and some Thread.Sleep, however, I want to write more appealing code if possible.

Answer 1

You really don't want to do this on Dispose() .

Consider a situation where your receiving application crashes and never ACK/NACK's one of the messages. You'll never receive enough responses to zero the counter, and when the dispose is called, possibly during GC or a using(...){...} block, it's going to hang indefinitely.

I'd suggest moving this functionality to another method which is called OnStop() .

And just keep your Dispose() method for cleaning up resources, closing connections etc...

Answer 2

How vital is it that those messages are sent? Generally, you should have some kind of Stop() method on your class to do what you are looking for. Doing this kind of action in the Dispose method is a little dangerous because Dispose methods shouldn't have the potential to throw errors or hang.

Also, make sure your worker thread is created with IsBackground set to true. That way, when your application closes, it is certain to complete as well.

Answer 3

The "thread safe" way to do it (and is how things like FileStream handle it) is to have a Flush() method that is blocking that writes out all the pending writes. Once you have that you just need to have Flush() be called inside the Dispose() method.

Now this will cause Dispose() to block, but you know what? We want dispose to block! If you do not block on dispose while work is still being done the last reference to the object could be released (very likely to happen inside a using block) and your object will be eligible for garbage collection before it is done processing.