Asynchronously wait for Task<T> to complete with timeout

Question

I want to wait for a Task<T> to complete with some special rules: If it hasn't completed after X milliseconds, I want to display a message to the user. And if it hasn't completed after Y milliseconds, I want to automatically request cancellation.

I can use Task.ContinueWith to asynchronously wait for the task to complete (i.e. schedule an action to be executed when the task is complete), but that doesn't allow to specify a timeout. I can use Task.Wait to synchronously wait for the task to complete with a timeout, but that blocks my thread. How can I asynchronously wait for the task to complete with a timeout?

Answer 1

How about this:

int timeout = 1000;
var task = SomeOperationAsync();
if (await Task.WhenAny(task, Task.Delay(timeout)) == task) {
    // task completed within timeout
} else { 
    // timeout logic
}

And here's a great blog post "Crafting a Task.TimeoutAfter Method" (from MS Parallel Library team) with more info on this sort of thing.

Addition: at the request of a comment on my answer, here is an expanded solution that includes cancellation handling. Note that passing cancellation to the task and the timer means that there are multiple ways cancellation can be experienced in your code, and you should be sure to test for and be confident you properly handle all of them. Don't leave to chance various combinations and hope your computer does the right thing at runtime.

int timeout = 1000;
var task = SomeOperationAsync(cancellationToken);
if (await Task.WhenAny(task, Task.Delay(timeout, cancellationToken)) == task)
{
    // Task completed within timeout.
    // Consider that the task may have faulted or been canceled.
    // We re-await the task so that any exceptions/cancellation is rethrown.
    await task;

}
else
{
    // timeout/cancellation logic
}

Answer 2

Here's a extension method version that incorporates cancellation of the timeout when the original task completes as suggested by Andrew Arnott in a comment to his answer.

public static async Task<TResult> TimeoutAfter<TResult>(this Task<TResult> task, TimeSpan timeout) {

    using (var timeoutCancellationTokenSource = new CancellationTokenSource()) {

        var completedTask = await Task.WhenAny(task, Task.Delay(timeout, timeoutCancellationTokenSource.Token));
        if (completedTask == task) {
            timeoutCancellationTokenSource.Cancel();
            return await task;  // Very important in order to propagate exceptions
        } else {
            throw new TimeoutException("The operation has timed out.");
        }
    }
}

Answer 3

You can use Task.WaitAny to wait the first of multiple tasks.

You could create two additional tasks (that complete after the specified timeouts) and then use WaitAny to wait for whichever completes first. If the task that completed first is your "work" task, then you're done. If the task that completed first is a timeout task, then you can react to the timeout (e.g. request cancellation).