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C#–Handling cancellation requests

When creating an async method in C#, I typically add the option to pass a CancellationToken. That is the easy part. But what if a consumer of your method uses this CancellationToken to request a cancellation? What is the proper way to cancel your code? Let's find out...

In most cases, it will be sufficient to pass the cancellation token to a lower-level API, but if we are providing the lowest-level API it is up to us to correctly handle the cancellation request.

The correct way to handle cancellations is by throwing an OperationCanceledException when a cancellation is requested.

To help you with this, a convenient ThrowIfCancellationRequested() method exists on the CancellationToken object.

Here are some other considerations to take into account when implementing cancellation:

  • Don’t cancel if you operation incures side-effects that would leave the system in an inconsistent state.
  • Don’t throw an OperationCancelledException when the work has already completed.
  • Start by checking if a cancellation was requested before executing your logic.
  • For work that is executed very quick, it it an option to not check the token(although calling CancellationToken.ThrowIfCancellationRequested() is pretty lightweight)
  • Check CancellationToken.CanBeCanceled when you can do your work more efficiently if you can assume you’ll never be canceled.
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