How to cancel a Task in await

Question

I m playing with these Windows 8 WinRT tasks  and I m trying to cancel a task using the method below  and it works to some point  The CancelNotification method DOES get called  which makes you think the task was cancelled  but in the background the task keeps running  then after it s completed  the status of the Task is always completed and never cancelled  Is there a way to completely halt the task when it s cancelled   private async void TryTask         CancellationTokenSource source   new CancellationTokenSource        source Token Register CancelNotification       source CancelAfter TimeSpan FromSeconds 1        var task   Task lt int gt  Factory StartNew      gt  slowFunc 1  2   source Token        await task                   if  task IsCompleted                MessageDialog md   new MessageDialog task Result ToString             await md ShowAsync              else               MessageDialog md   new MessageDialog  Uncompleted            await md ShowAsync             private int slowFunc int a  int b        string someString   string Empty      for  int i   0  i  lt  200000  i                  someString     a              return a   b     private void CancelNotification

User · Answer

I just want to add to the already accepted answer. I was stuck on this, but I was going a different route on handling the complete event. Rather than running await, I add a completed handler to the task.

Comments.AsAsyncAction().Completed += new AsyncActionCompletedHandler(CommentLoadComplete);

Where the event handler looks like this

private void CommentLoadComplete(IAsyncAction sender, AsyncStatus status )
{
    if (status == AsyncStatus.Canceled)
    {
        return;
    }
    CommentsItemsControl.ItemsSource = Comments.Result;
    CommentScrollViewer.ScrollToVerticalOffset(0);
    CommentScrollViewer.Visibility = Visibility.Visible;
    CommentProgressRing.Visibility = Visibility.Collapsed;
}

With this route, all the handling is already done for you, when the task is cancelled it just triggers the event handler and you can see if it was cancelled there.

User · Answer

One case which hasn t been covered is how to handle cancellation inside of an async method  Take for example a simple case where you need to upload some data to a service get it to calculate something and then return some results   public async Task lt Results gt  ProcessDataAsync MyData data        var client   await GetClientAsync        await client UploadDataAsync data       await client CalculateAsync        return await client GetResultsAsync        If you want to support cancellation then the easiest way would be to pass in a token and check if it has been cancelled between each async method call  or using ContinueWith   If they are very long running calls though you could be waiting a while to cancel  I created a little helper method to instead fail as soon as canceled   public static class TaskExtensions       public static async Task lt T gt  WaitOrCancel lt T gt  this Task lt T gt  task  CancellationToken token                token ThrowIfCancellationRequested            await Task WhenAny task  token WhenCanceled             token ThrowIfCancellationRequested             return await task             public static Task WhenCanceled this CancellationToken cancellationToken                var tcs   new TaskCompletionSource lt bool gt             cancellationToken Register s   gt    TaskCompletionSource lt bool gt  s  SetResult true   tcs           return tcs Task            So to use it then just add  WaitOrCancel token  to any async call   public async Task lt Results gt  ProcessDataAsync MyData data  CancellationToken token        Client client      try               client   await GetClientAsync   WaitOrCancel token           await client UploadDataAsync data  WaitOrCancel token           await client CalculateAsync   WaitOrCancel token           return await client GetResultsAsync   WaitOrCancel token             catch  OperationCanceledException                if  client    null              await client CancelAsync            throw            Note that this will not stop the Task you were waiting for and it will continue running  You ll need to use a different mechanism to stop it  such as the CancelAsync call in the example  or better yet pass in the same CancellationToken to the Task so that it can handle the cancellation eventually  Trying to abort the thread isn t recommended

User · Answer

Read up on Cancellation  which was introduced in  NET 4 0 and is largely unchanged since then  and the Task-Based Asynchronous Pattern  which provides guidelines on how to use CancellationToken with async methods   To summarize  you pass a CancellationToken into each method that supports cancellation  and that method must check it periodically   private async Task TryTask       CancellationTokenSource source   new CancellationTokenSource      source CancelAfter TimeSpan FromSeconds 1      Task lt int gt  task   Task Run      gt  slowFunc 1  2  source Token   source Token          A canceled task will raise an exception when awaited     await task     private int slowFunc int a  int b  CancellationToken cancellationToken      string someString   string Empty    for  int i   0  i  lt  200000  i            someString     a       if  i   1000    0        cancellationToken ThrowIfCancellationRequested           return a   b

User · Answer

Or  in order to avoid modifying slowFunc  say you don t have access to the source code for instance    var source   new CancellationTokenSource      original code source Token Register CancelNotification     original code source CancelAfter TimeSpan FromSeconds 1      original code var completionSource   new TaskCompletionSource lt object gt       New code source Token Register      gt  completionSource TrySetCanceled       New code var task   Task lt int gt  Factory StartNew      gt  slowFunc 1  2   source Token     original code    original code  await task    await Task WhenAny task  completionSource Task     New code   You can also use nice extension methods from https   github com StephenCleary AsyncEx and have it looks as simple as   await Task WhenAny task  source Token AsTask

[c#] How to cancel a Task in await?

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