How to Wait for Canceled Task to Finish?

Question

I obviously don\'t know what I\'m doing when it comes to parallel programming with .NET 4.0. I have a simple Windows app that starts a task to do some mindless work (outputt

Answer 1

You would normally just use Task.Wait (instead of WaitAll), as it's a single task. and then handled the exception appropriately:

private void stopButton_Click(object sender, EventArgs e)
{
    cts.Cancel();
    try
    {
        t.Wait();  // This will throw
    }
    catch (AggregateException ae)
    {
       ae.Handle(ce => true);
    }

    statusTextBox.Text = "Output ended.";
}

When you cancel a Task, the OperationCanceledException will get wrapped into an AggregateException and be thrown as soon as you call Wait() or try to get the Task's Result (if it's a Task).

---

Purely for your information - This is one place, especially given what you're doing here, where C# 5 simplifies things. Using the new async support, you can write this as:

// No need for "t" variable anymore 
// private Task t;


private async void startButton_Click(object sender, EventArgs e)
{
   statusTextBox.Text = "Output started.";

   // Create the cancellation token source.
   cts = new CancellationTokenSource();

   try
   {
      // Create & start worker task.
      await Task.Run(() => DoWork(cts.Token));
      statusTextBox.Text = "Output ended.";
   }
   catch(OperationCanceledException ce) 
   {
      // Note that we get "normal" exception handling
      statusTextBox.Text = "Operation canceled.";
   }
}

private void stopButton_Click(object sender, EventArgs e)
{
   // Just cancel the source - nothing else required here
   cts.Cancel();
}