Why debug is different from running without debugging (Cancelling a Task)?

Question

I made a program to test Parallel programming in C# 4.0. However, there is a problem and I can't figure out why.

I write a method

    private void compute(int startValue, int endValue, ConcurrentBag<Pair> theList)
    {
        try
        {
            Task computation = Task.Factory.StartNew(() => { findFriendlyNumbers(startValue, endValue, theList, tokenSource.Token); }, tokenSource.Token);
            Task.WaitAll(computation);
            StringBuilder builder = new StringBuilder();
            foreach (Pair p in theList)
            {
                builder.AppendLine(p.ToString());
            }

            this.textBlockResult.Dispatcher.Invoke(new Action(() =>
            {
                this.textBlockResult.Text = builder.ToString();
                this.progressBar1.Visibility = System.Windows.Visibility.Hidden;
            }));
        }
        catch (AggregateException aEx)
        {
            MessageBox.Show("Entering");  //For debug, but never runs
            aEx.Handle(handleCancelling);
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    }

Which is run on a separate thread (other than the UI thread).

And the function (simplified)

        private void findFriendlyNumbers(int start, int end, ConcurrentBag<Pair> list, CancellationToken token)
    {
        //some initialization and computation

        for (int i = start; i <= end; i++)
        {
            //check whether it's cancelled
            token.ThrowIfCancellationRequested();

            //some computation
        }
    }

The problem is, when the tokenSource is canceled, there will be an error "OperationCanceledException is not handled by user code", as if the catch block doesn't exist. I am not sure why, because my code is similar to that in textbook and also in MSDN.

Thank you.

EDIT: I actually wrote a similar program less than a month ago, and everything was fine then. Today I tried to run it again, the same problem happened. I installed Microsoft Visual Web Developer 2010 Express after I finished the program, and I am not sure whether this is the reason. I don't get it, the same code, different results.

EDIT: I thought about this problem and found where is wrong. In the past, I used to use "run without debugging" while I use debug now. Running without debugging solves the problem. And I will appreciate it if someone tells me why debug is different from "run without debug".

Answer 1

You're passing the cancellation token in to the TaskFactory.StartNew method. This makes the Task treat it specially: as an indication of cancellation instead of an error.

If you don't pass the token as a parameter to TaskFactory.StartNew, then it will be treated as an error and caught. If you do keep it as a parameter, then you need to check for cancellation using Task.IsCanceled instead of an exception.

Side note: it's best not to use Invoke to synchronize to the UI thread. I have an example of Task objects properly doing UI progress updates on my blog.