CODEWITHSUNDEEP
c#.netasync-awaittask-parallel-librarytask
Tony
Tony

Reputation: 1937

What is the best way to end a task to prevent run-away

I have created the function below which would wait for all the tasks to complete or raise an exception when a cancellation or time-out occurs. 

public static async Task WhenAll(
    IEnumerable<Task> tasks, 
    CancellationToken cancellationToken,
    int millisecondsTimeOut)
{
    Task timeoutTask = Task.Delay(millisecondsTimeOut, cancellationToken);
    Task completedTask = await Task.WhenAny(
        Task.WhenAll(tasks), 
        timeoutTask
    );
    if (completedTask == timeoutTask)
    {
        throw new TimeoutException();
    }
}

If all the tasks finished before a long time-out (i.e. millisecondsTimeOut = 60,000), would timeoutTask be staying around until 60 seconds has elapsed even after the function returns? If yes, what is the best way to fix the runaway problem?

Upvotes: 3

Views: 1109

Answers (1)

i3arnon
i3arnon

Reputation: 116538

Yes, the timeoutTask would hang around until that timeout is over (or the CancellationToken is canceled).

You can fix it by passing in a different CancellationToken you get from a new CancellationTokenSource you create using CancellationTokenSource.CreateLinkedTokenSource and cancelling at the end. You should also await the completed task, otherwise you aren't really observing exceptions (or cancellations):

public static async Task WhenAll(
    IEnumerable<Task> tasks,
    CancellationToken cancellationToken,
    int millisecondsTimeOut)
{
    var cancellationTokenSource = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
    var timeoutTask = Task.Delay(millisecondsTimeOut, cancellationTokenSource.Token);
    var completedTask = await Task.WhenAny(Task.WhenAll(tasks), timeoutTask);
    if (completedTask == timeoutTask)
    {
        throw new TimeoutException();
    }

    cancellationTokenSource.Cancel();
    await completedTask;
}

However, I think there's a simpler way to achieve what you want, if you don't need to distinguish between a TimeoutException and TaskCancelledException. You just add a continuation that is cancelled when the CancellationToken is cancelled or when the timeout is over:

public static Task WhenAll(
    IEnumerable<Task> tasks,
    CancellationToken cancellationToken,
    int millisecondsTimeOut)
{
    var cancellationTokenSource = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
    cancellationTokenSource.CancelAfter(millisecondsTimeOut);

    return Task.WhenAll(tasks).ContinueWith(
        _ => _.GetAwaiter().GetResult(), 
        cancellationTokenSource.Token,
        TaskContinuationOptions.ExecuteSynchronously, 
        TaskScheduler.Default);
}

Upvotes: 2

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