I have interactive task which in "worst" scenario is not executed at all, thus it is represented by TaskCompletionSource.
I would like to wait for either this task completes, or token which I received is cancelled -- whichever happens first. Perfect tool for such job would be Task.WhenAny, the only problem is it takes only tasks, and I have one Task and one CancellationToken.
How to wait (asynchronously, like Task.WhenAny) for the first event triggered -- completed task, or cancelled token?
async Task MyCodeAsync(CancellationToken token)
{
var tcs = new TaskCompletionSource<UserData>(); // represents interactive part
await Task.WhenAny(tcs.Task, token); // imaginary call
UserData data = tcs.Task.Result; // user interacted, let's continue
...
}
I don't create/manage token, so I cannot change it. I have to deal with it.
Update: For such particular case one could use Register method on token to cancel the TaskCompletionSource. For more general method please see Matthew Watson answer.
Here is an extension method that transforms a CancellationToken to a Task or Task<TResult>. The returned task will complete as cancelled immediately after the CancellationToken receives a cancellation request.
static class CancellationTokenExtensions
{
public static Task AsTask(this CancellationToken token)
{
return new Task(() => throw new InvalidOperationException(), token);
}
public static Task<TResult> AsTask<TResult>(this CancellationToken token)
{
return new Task<TResult>(() => throw new InvalidOperationException(), token);
}
}
Usage example. Just await any task:
await Task.WhenAny(tcs.Task, token.AsTask());
...or await and get the result in the same line as well:
var data = await Task.WhenAny(tcs.Task, token.AsTask<UserData>()).Unwrap();
The InvalidOperationException is thrown just in case, to ensure that the task of the CancellationToken will never run to completion. Its Status can only be Created, Canceled or Faulted.
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