TaskCanceledException when calling Task.Delay with a CancellationToken in an keyboard event

Question

I am trying to delay the processing of a method (SubmitQuery() in the example) called from an keyboard event in WinRT until there has been no further events for a time period (500ms in this case).

I only want SubmitQuery() to run when I think the user has finished typing.

Using the code below, I keep getting a System.Threading.Tasks.TaskCanceledException when Task.Delay(500, cancellationToken.Token); is called. What am I doing wrong here please?

CancellationTokenSource cancellationToken = new CancellationTokenSource();  private async void SearchBox_QueryChanged(SearchBox sender, SearchBoxQueryChangedEventArgs args) {          cancellationToken.Cancel();         cancellationToken = new CancellationTokenSource();      await Task.Delay(500, cancellationToken.Token);      if (!cancellationToken.IsCancellationRequested)     {         await ViewModel.SubmitQuery();     } } 

Answer 1

If you add ContinueWith() with an empty action, the exception isn't thrown. The exception is caught and passed to the tsk.Exception property in the ContinueWith(). But It saves you from writing a try/catch that uglifies your code.

await Task.Delay(500, cancellationToken.Token).ContinueWith(tsk => { }); 

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UPDATE:

Instead of writing code to handle the exception, a boolean would be much cleaner. This is only preferred when a delay cancel is expected!. One way is to create a helper class (Although I don't like helper classes much)

namespace System.Threading.Tasks {     public static class TaskDelay     {         public static Task<bool> Wait(TimeSpan timeout, CancellationToken token) =>             Task.Delay(timeout, token).ContinueWith(tsk => tsk.Exception == default);          public static Task<bool> Wait(int timeoutMs, CancellationToken token) =>             Task.Delay(timeoutMs, token).ContinueWith(tsk => tsk.Exception == default);     } } 

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For example:

var source = new CancellationTokenSource();  if(!await TaskDelay.Wait(2000, source.Token)) {     // The Delay task was canceled. } 

(don't forget to dispose the source)