Task.WhenAny with cancellation of the non completed tasks and timeout

Question

In my app I am creating some concurrent web requests and I am satisfied when any one of them completes, so I am using the method Task.WhenAny:

var urls = new         


        

Answer 1

Simply pass to all of your tasks the same cancellation token, something like this:

CancellationTokenSource cts = new CancellationTokenSource();
CancellationToken ct = cts.Token;
// here you specify how long you want to wait for task to finish before cancelling
int timeout = 5000;
cts.CancelAfter(timeout);
// pass ct to all your tasks and start them
await Task.WhenAny(/* your tasks here */);
// cancel all tasks
cts.Cancel();

Also, you need to read this thread to be aware of how to use CancellationToken correctly: When I use CancelAfter(), the Task is still running

Answer 2

Update: better solution based on Stephen Cleary's answer and MSDN and svick's answer:

CancellationTokenSource source = new CancellationTokenSource();
source.CancelAfter(TimeSpan.FromSeconds(1));

var tasks = urls.Select(url => Task.Run( async () => 
{
    using (var webClient = new WebClient())
    {
        token.Register(webClient.CancelAsync);
        var result = (Url: url, Data: await webClient.DownloadStringTaskAsync(url));
        token.ThrowIfCancellationRequested();
        return result.Url;
    }
}, token)).ToArray();

string url;
try
{
    // (A canceled task will raise an exception when awaited).
    var firstTask = await Task.WhenAny(tasks);
    url = (await firstTask).Url;
}   
catch (AggregateException ae) {
   foreach (Exception e in ae.InnerExceptions) {
      if (e is TaskCanceledException)
         Console.WriteLine("Timeout: {0}", 
                           ((TaskCanceledException) e).Message);
      else
         Console.WriteLine("Exception: " + e.GetType().Name);
   }
}

---

non-optimal solution

The timeout can be solved by adding a task that just waits and completes after given time. Then you check which task completed first, if it is the waiting one, then timeout effectively occurred.

Task timeout = Task.Delay(10000);
var firstTask = await Task.WhenAny(tasks.Concat(new Task[] {timeout}));
if(firstTask == timeout) { ... } //timed out
source.Cancel();