Timeout extension method for Task<T>

TaskTimeout.cs

 // I don't remember where this is taken/adapted from. 
 public static class AsyncExtensions
   {
       public static async Task<T> TimeoutAfter<T>(this Task<T> task, TimeSpan timeout, CancellationTokenSource cancellationTokenSource = null)
       {
           if (task == await Task.WhenAny(task, Task.Delay(timeout)))
               return await task;
           else
           {
               if (cancellationTokenSource != null)
                   cancellationTokenSource.Cancel();

               throw new TimeoutException();
           }
       }
   }