Cancelling an HttpClient Request - Why is TaskCanceledException.CancellationToken.IsCancellationRequested false?

Question

Given the following code:

var cts = new CancellationTokenSource();

try 
{
    // get a "hot" task
    var task = new HttpClient().GetAsync("http://www.google.com", cts.Token);

    // request cancellation
    cts.Cancel();

    await task;

    // pass:
    Assert.Fail("expected TaskCanceledException to be thrown");
}
catch (TaskCanceledException ex) 
{
    // pass:
    Assert.IsTrue(cts.Token.IsCancellationRequested,
        "expected cancellation requested on original token");

    // fail:
    Assert.IsTrue(ex.CancellationToken.IsCancellationRequested,
        "expected cancellation requested on token attached to exception");
}

I would expect ex.CancellationToken.IsCancellationRequested to be true inside the catch block, but it is not. Am I misunderstanding something?

Answer 1

That's the case because HttpClient internally (in SendAsync) is using a TaskCompletionSource to represent the async operation. It returns TaskCompletionSource.Task and that's the task you await on.

It then calls base.SendAsync and registers a continuation on the returned task that cancels/completes/faults the TaskCompletionSource's task accordingly.

In the case of cancellation it uses TaskCompletionSource.TrySetCanceled which associates the canceled task with a new CancellationToken (default(CancellationToken)).

You can see that by looking at the TaskCanceledException. On top of ex.CancellationToken.IsCancellationRequested being false ex.CancellationToken.CanBeCanceled is also false, meaning that this CancellationToken can never be canceled as it wasn't created using a CancellationTokenSource.

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IMO it should be using TaskCompletionSource.TrySetCanceled(CancellationToken) instead. That way the TaskCompletionSource will be associated with the CancellationToken passed in by the consumer and not simply the default CancellationToken. I think it's a bug (though a minor one) and I submitted an issue on connect about it.