Question

in my class i use a BackgroundWorker. at some point i need to cancel the asynchronous operation that can be in progress and start another one immediately. the code follows. one thing that i am not sure about is the race condition that can occur if the worker completes right before i assign my lambda to RunWorkerCompleted event. if this happens my lambda will never get called. the comment in the code shows this place. any comments on how to handle this?

thanks
konstantin

if (this.worker.IsBusy)
{
    RunWorkerCompletedEventHandler f = null;

    f = (s, v) =>
    {
        this.RunWorkerCompleted -= f;
        this.worker.RunWorkerAsync();
    };

    // what if worker completes right before the following statement?
    this.worker.RunWorkerCompleted += f;
    this.worker.CancelAsync();
}
else
{
    this.worker.RunWorkerAsync();
}

Answer 1

As long as this code runs on the main thread then there is no race. A BGW can only complete when the RunWorkerCompleted event handler finished running. The handler cannot start running until the main thread re-enters the message loop.

There’s another kind of race though, induced by the else clause. You let the BGW start without a RunWorkerCompleted event handler. Now it can complete asynchronously since it won’t be blocked. Always subscribe the event, test e.Cancelled to know what happened.