How to cancel a Task in await?

Question

I'm playing with these Windows 8 WinRT tasks, and I'm trying to cancel a task using the method below, and it works to some point. The CancelNotification method DOES get called, which makes you think the task was cancelled, but in the background the task keeps running, then after it's completed, the status of the Task is always completed and never cancelled. Is there a way to completely halt the task when it's cancelled?

private async void TryTask()
{
    CancellationTokenSource source = new CancellationTokenSource();
    source.Token.Register(CancelNotification);
    source.CancelAfter(TimeSpan.FromSeconds(1));
    var task = Task<int>.Factory.StartNew(() => slowFunc(1, 2), source.Token);

    await task;            

    if (task.IsCompleted)
    {
        MessageDialog md = new MessageDialog(task.Result.ToString());
        await md.ShowAsync();
    }
    else
    {
        MessageDialog md = new MessageDialog("Uncompleted");
        await md.ShowAsync();
    }
}

private int slowFunc(int a, int b)
{
    string someString = string.Empty;
    for (int i = 0; i < 200000; i++)
    {
        someString += "a";
    }

    return a + b;
}

private void CancelNotification()
{
}

Answer 1

Read up on Cancellation (which was introduced in .NET 4.0 and is largely unchanged since then) and the Task-Based Asynchronous Pattern, which provides guidelines on how to use CancellationToken with async methods.

To summarize, you pass a CancellationToken into each method that supports cancellation, and that method must check it periodically.

private async Task TryTask()
{
  CancellationTokenSource source = new CancellationTokenSource();
  source.CancelAfter(TimeSpan.FromSeconds(1));
  Task<int> task = Task.Run(() => slowFunc(1, 2, source.Token), source.Token);

  // (A canceled task will raise an exception when awaited).
  await task;
}

private int slowFunc(int a, int b, CancellationToken cancellationToken)
{
  string someString = string.Empty;
  for (int i = 0; i < 200000; i++)
  {
    someString += "a";
    if (i % 1000 == 0)
      cancellationToken.ThrowIfCancellationRequested();
  }

  return a + b;
}