.net - Timeout pattern on task-based asynchronous method in C#

Question

As far as I know, there're two possible patterns to implement a timeout to task-based asynchronous methods:

Built-in timeout

public Task DoStuffAsync(TimeSpan timeout)

This approach is harder to implement because it's not easy to implement a global timeout for the entire call stack. For example, a Web API controller receives an HTTP request and it calls DoStuffAsync, and the caller wants a global timeout of 3 seconds.

That is, each inner async method call will need to receive the subtract of already used time...

No built-in timeout

public Task DoStuffAsync(CancellationToken cancellationToken)

..........

CancellationTokenSource cancellationSource = new CancellationTokenSource();
Task timeoutTask = Task.Delay(3000);

if(await Task.WhenAny(DoStuffAsync(cancellationTokenSource), timeoutTask) == timeoutTask)
{
     cancellationSource.Cancel();

     throw new TimeoutException();
}

This seems to be the most reliable and easy to implement pattern. The first caller defines a global timeout, and if it time outs, all pending operations will be cancelled. Also, it provides a cancellation token to the immediate caller and inner calls will share the same cancellation token reference. Thus, if the top caller time outs, it will be able to cancel any working thread.

The whole question

Is there any pattern that I'm missing or, am I in the right way if I develop APIs using the no built-in timeout?

Answer 1

While you can reuse WithCancellation for both cancellations and timeouts I think it's an overkill for what you need.

A simpler and clearer solution for an async operation timeout would be to await both the actual operation and a timeout task using Task.WhenAny. If the timeout task completes first, you got yourself a timeout. Otherwise, the operation completed successfully:

public static async Task<TResult> WithTimeout<TResult>(this Task<TResult> task, TimeSpan timeout)
{
    if (task == await Task.WhenAny(task, Task.Delay(timeout)))
    {
        return await task;
    }
    throw new TimeoutException();
}

Usage:

try
{
    await DoStuffAsync().WithTimeout(TimeSpan.FromSeconds(5));
}
catch (TimeoutException)
{
    // Handle timeout.
}

---

If you prefer to not throw an exception (as I do) it's even simpler, just return the default value:

public static Task<TResult> WithTimeout<TResult>(this Task<TResult> task, TimeSpan timeout)
{
    var timeoutTask = Task.Delay(timeout).ContinueWith(_ => default(TResult), TaskContinuationOptions.ExecuteSynchronously);
    return Task.WhenAny(task, timeoutTask).Unwrap();
}