What is the use-case for `CancellationTokenSource.TryReset`?

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CancellationTokenSource has a TryReset() member. The documentation appears so strict as to make me wonder why it exists.

  • It will only work if no-one has already called Cancel() on it.
  • It is only valid to use when any previous asynchronous operations issued with its token have completed, which means (I think) that I would need to hang on to my Task object to track whether it is still running
  • It is not safe to call when someone else is trying to cancel the operation, requiring

So why would people bother with TryReset? Why not simply create a brand new CancellationTokenSource for every asynchronous operation and then throw it away after the operation completes and cancellation is no longer needed? Is CancellationTokenSource a terribly expensive object to create?

cancellationtokensource task-parallel-library .net c#
1 Answers

Let me quote here the Background and Motivation section of the issue

When a library or framework exposes a CancellationToken that usually does not get canceled (e.g. HttpContext.RequestAborted) they often still need to dispose the backing CancellationTokenSource (CTS) after an operation completes rather than reusing it in order to account for callers that might never dispose their registrations.

To use the RequestAborted example, Kestrel disposes the backing CTS after every request where the RequestAborted token is accessed. If Kestrel attempted to reuse the CTS backing the RequestAborted token for future requests in order to reduce allocations, it would risk leaking any undisposed registrations and triggering the undisposed registrations when unrelated requests are aborted.

Another scenario where people want to be able to "reset" a CTS is after calling CancelAfter(). This can already be achieved by calling CancelAfter(Timeout.Infinite), but that's not necessarily obvious unless you read the docs. TryReset() is something that would immediately make sense in this scenario when looking at intellisense completions.

Another benefit is that it's immediately obvious if resetting failed. If you try resetting a timeout with CancelAfter(), you have to check after the call to verify their was no cancellation before or during the call causing CancelAfter() to no-op. This is demonstrated in the second HttpClient usage example.

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