How to reset a CancellationToken properly?

Question

I have been playing round with the Async CTP this morning and have a simple program with a button and a label. Click the button and it starts updating the label, stop the button it stops writing to the label. However, I'm not sure how to reset the CancellationTokenSource so that I can restart the process.

My code:

public partial class MainWindow : Window {     CancellationTokenSource cts = new CancellationTokenSource();     public MainWindow()     {         InitializeComponent();         button.Content = "Start";     }      async Task DoWork(CancellationToken cancelToken)     {         int i = 0;         while (!cancelToken.IsCancellationRequested)         {             label.Content = i++.ToString();             await TaskEx.Delay(50, cancelToken);         }     }      private void Button_Click(object sender, RoutedEventArgs e)     {         if (button.Content == "Start")         {             button.Content = "Stop";             DoWork(cts.Token);         }         else         {             button.Content = "Start";             cts.Cancel();         }     } } 

Answer 1

You need to recreate the CancellationTokenSource - there is no way to "reset" this once you set it.

This could be as simple as:

private void Button_Click(object sender, RoutedEventArgs e) {     if (button.Content == "Start")     {         button.Content = "Stop";         cts.Dispose(); // Clean up old token source....         cts = new CancellationTokenSource(); // "Reset" the cancellation token source...         DoWork(cts.Token);     }     else     {         button.Content = "Start";         cts.Cancel();     } } 

Answer 2

I had the same problem and I figured it out that the best way to solve it is to create cancellation token source newly just before calling the method.

this is what I do on my start button click:

cancellationTokenSource = new CancellationTokenSource(); cancellationToken = cancellationTokenSource.Token; Task.Factory.StartNew(StartUpload, cancellationToken); 

I change the caption for the same button to cancel and when a click occurs on cancel, I call

cancellationTokenSource.Cancel(); 

Here is the full code:

if (button3.Text != "&Start Upload") {     cancellationTokenSource.Cancel(); } else {     try     {         cancellationTokenSource = new CancellationTokenSource();         cancellationToken = cancellationTokenSource.Token;         Task.Factory.StartNew(StartUpload, cancellationToken);     }     catch (AggregateException ex)     {         var builder = new StringBuilder();         foreach (var v in ex.InnerExceptions)             builder.Append("\r\n" + v.InnerException);         MessageBox.Show("There was an exception:\r\n" + builder.ToString());     }     catch (Exception ex)     {         MessageBox.Show(ex.Message);     } }