How to: Register Callbacks for Cancellation Requests

The following example shows how to register a delegate that will be invoked when a IsCancellationRequested property becomes true due to a call to Cancel on the object that created the token. Use this technique for cancelling asynchronous operations that do not natively support the unified cancellation framework, and for unblocking methods that might be waiting for an asynchronous operation to finish.


When "Just My Code" is enabled, Visual Studio in some cases will break on the line that throws the exception and display an error message that says "exception not handled by user code." This error is benign. You can press F5 to continue from it, and see the exception-handling behavior that is demonstrated in the examples below. To prevent Visual Studio from breaking on the first error, just uncheck the "Just My Code" checkbox under Tools, Options, Debugging, General.


In the following example, the CancelAsync method is registered as the method to be invoked when cancellation is requested through the cancellation token.

using System;
using System.Net;
using System.Threading;
using System.Threading.Tasks;

class CancelWithCallback
   static void Main()
      var cts = new CancellationTokenSource();
      var token = cts.Token;
      // Start cancelable task.
      Task t = Task.Run( () => {
                    WebClient wc = new WebClient();

                    // Create an event handler to receive the result.
                    wc.DownloadStringCompleted += (obj, e) => {
                               // Check status of WebClient, not external token.
                               if (!e.Cancelled) {
                                  Console.WriteLine("The download has completed:\n");
                                  Console.WriteLine(e.Result + "\n\nPress any key.");
                               else {
                                  Console.WriteLine("The download was canceled.");

                    // Do not initiate download if the external token
                    // has already been canceled.
                    if (!token.IsCancellationRequested) {
                       // Register the callback to a method that can unblock.
                       using (CancellationTokenRegistration ctr = token.Register(() => wc.CancelAsync()))
                          Console.WriteLine("Starting request\n");
                          wc.DownloadStringAsync(new Uri(""));
               }, token);

      Console.WriteLine("Press 'c' to cancel.\n");
      char ch = Console.ReadKey().KeyChar;
      if (ch == 'c')

      Console.WriteLine("Press any key to exit.");
Imports System.Threading
Imports System.Threading.Tasks
Imports System.Net

Class CancelWithCallback
    Shared Sub Main()
        Dim cts As New CancellationTokenSource()
        Dim token As CancellationToken = cts.Token
        ' Start cancelable task.
        Dim t As Task = Task.Run(
              Dim wc As New WebClient()
              ' Create an event handler to receive the result.
              AddHandler wc.DownloadStringCompleted,
                         Sub(obj, e)
                            ' Check status of WebClient, not external token.
                            If Not e.Cancelled Then
                               Console.WriteLine("The download has completed:" + vbCrLf)
                               Console.WriteLine(e.Result + vbCrLf + vbCrLf + "Press any key.")
                               Console.WriteLine("Download was canceled.")
                            End If
                          End Sub
              Using ctr As CancellationTokenRegistration = token.Register(Sub() wc.CancelAsync())
                 Console.WriteLine("Starting request..." + vbCrLf)
                 wc.DownloadStringAsync(New Uri(""))
              End Using
           End Sub, token)

        Console.WriteLine("Press 'c' to cancel." + vbCrLf)
        Dim ch As Char = Console.ReadKey().KeyChar

        If ch = "c"c Then
        End If
        Console.WriteLine("Press any key to exit.")
    End Sub
End Class

If cancellation has already been requested when the callback is registered, the callback is still guaranteed to be called. In this particular case, the CancelAsync method will do nothing if no asynchronous operation is in progress, so it is always safe to call the method.

See also