ICancellationTokenSource

[timcassell]

In the same vein as IValueTaskSource, I would like CancellationToken to be updated to allow an ICancellationTokenSource passed to a new constructor.

Reason: In my ProtoPromise library, I have a custom CancelationSource which is a struct backed by a class that gets pooled for reuse when it is disposed. As it is now, System.Threading.CancellationTokenSource is a class that cannot be reused (much like TaskCompletionSource), and thus subject to garbage collection.

Async iterators added System.Threading.CancellationToken as part of the contract, so I would like to be able to freely convert my custom CancelationToken to System.Threading.CancellationToken without having to allocate a new System.Threading.CancellationTokenSource.

[stephentoub]

Thanks for the suggestion, but we've discussed this before and don't want to do it. CancellationToken has a strong guarantee that once it transitions to having cancellation requested, it'll never transition back, and such use would break that guarantee. You can, of course, reuse a CTS as many times as you'd like if it hasn't yet had cancellation requested (and if it has, the associated costs almost always are orders of magnitude larger than what you'd potentially save by pooling), subject to whatever guarantees your own code provides, and we added TryReset in .NET 6 to make that easier, in particular in the face of timeouts.

[timcassell]

@stephentoub I see, that makes a lot of sense. Certainly the new TryReset should make it much better than allocating new every time, especially since cancelations are the exception, not the rule.

Even better would be the ability to use custom cancelation tokens with async iterators, but I can't imagine a clean way of doing that. 😛

In case you were wondering, my own CancelationToken has a (Try)Retain/Release for the consumer to protect its integrity for the duration it's needed, though in practice it's generally not needed if the producer only disposes when the requested operation completes/fails.

Thanks for the response!