Question

I need to cancel a background task started using ThreadPool.QueueUserWorkItem(…). I know a BackgroundWorker has constructs especially for this sort of thing, but I believe it’s overkill in this case, since no user interface is involved. By cancellation, I simply mean force the completion of the callback method.

What are the pitfalls of adding something like the following to my class?

// Cancellation Property.
private bool _canceled;
public bool CancelTask
{
    get { return _canceled; }
    set { _canceled = value; }
}

public void DoSomeTask()
{
    int iterations = 50;
    ThreadPool.QueueUserWorkItem(new WaitCallback(ThreadPoolCallback), iterations);
}

private void ThreadPoolCallback(object state)
{
    if (_canceled)
        return; // don't even start.

    int iterations = (int)state;
    for (int i = 0; !_canceled && i < iterations; i++)
    {
        //
        // do work ...
        //

        // This allows you to cancel in the middle of an iteration...
        if (_canceled)
            break;
    }
}

Is there a better way?

Answer 1

I’d use a method CancelTask() rather than a property. The point is that callers should be able to cancel a task, but no one should be able to un-cancel a task.

Then you need to be sure that the read and the write of _cancelled have the appropriate memory barriers otherwise one thread might not ever observe the change made by the other thread. For this I’d use Thread.VolatileWrite (inside CancelTask) and Thread.VolatileRead (inside your loop)