CancellationTokenSource.Cancel Method

Communicates a request for cancellation.

Namespace:  System.Threading
Assemblies:   System.Threading.Tasks (in System.Threading.Tasks.dll)
  mscorlib (in mscorlib.dll)

public void Cancel()

ExceptionCondition
ObjectDisposedException

This CancellationTokenSource has been disposed.

AggregateException

An aggregate exception containing all the exceptions thrown by the registered callbacks on the associated CancellationToken.

The associated CancellationToken will be notified of the cancellation and will transition to a state where IsCancellationRequested returns true.

Any callbacks or cancelable operations registered with the CancellationToken will be executed.

Cancelable operations and callbacks registered with the token should not throw exceptions.

However, this overload of Cancel will aggregate any exceptions thrown into an AggregateException, such that one callback throwing an exception will not prevent other registered callbacks from being executed.

The ExecutionContext that was captured when each callback was registered will be reestablished when the callback is invoked.

The following example uses a random number generator to emulate a data collection application that reads 10 integral values from eleven different instruments. A value of zero indicates that the measurement has failed for one instrument, in which case the operation should be cancelled and no overall mean should be computed.

To handle the possible cancellation of the operation, the example instantiates a CancellationTokenSource object that generates a cancellation token which is passed to a TaskFactory object. The TaskFactory object in turn passes the cancellation token to each of the tasks responsible for collecting readings for a particular instrument. The TaskFactory.ContinueWhenAll<TAntecedentResult, TResult>(Task<TAntecedentResult>[], Func<Task<TAntecedentResult>[], TResult>, CancellationToken) method is called to ensure that the mean is computed only after all readings have been gathered successfully. If a task has not because it has been cancelled, the call to the TaskFactory.ContinueWhenAll method throws an exception.

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
   public static void Main()
   {
      // Define the cancellation token.
      CancellationTokenSource source = new CancellationTokenSource();
      CancellationToken token = source.Token;

      Random rnd = new Random();
      Object lockObj = new Object();

      List<Task<int[]>> tasks = new List<Task<int[]>>();
      TaskFactory factory = new TaskFactory(token);
      for (int taskCtr = 0; taskCtr <= 10; taskCtr++) {
         int iteration = taskCtr + 1;
         tasks.Add(factory.StartNew( () => {
                                       int value;
                                       int[] values = new int[10];
                                       for (int ctr = 1; ctr <= 10; ctr++) {
                                          lock (lockObj) {
                                             value = rnd.Next(0,101);
                                          }
                                          if (value == 0) { 
                                             source.Cancel();
                                             Console.WriteLine("Cancelling at task {0}", iteration);
                                             break;
                                          }   
                                          values[ctr-1] = value; 
                                       }
                                       return values;
                                    }, token));   

      }
      try {
         Task<double> fTask = factory.ContinueWhenAll(tasks.ToArray(), 
                                                      (results) => {
                                                         Console.WriteLine("Calculating overall mean...");
                                                         long sum = 0;
                                                         int n = 0; 
                                                         foreach (var t in results) {
                                                            foreach (var r in t.Result) {
                                                                  sum += r;
                                                                  n++;
                                                               }
                                                         }
                                                         return sum/(double) n;
                                                      } , token);
         Console.WriteLine("The mean is {0}.", fTask.Result);
      }   
      catch (AggregateException ae) {
         foreach (Exception e in ae.InnerExceptions) {
            if (e is TaskCanceledException)
               Console.WriteLine("Unable to compute mean: {0}", 
                                 ((TaskCanceledException) e).Message);
            else
               Console.WriteLine("Exception: " + e.GetType().Name);
         }
      }
      finally {
         source.Dispose();
      }
   }
}
// Repeated execution of the example produces output like the following: 
//       Cancelling at task 5 
//       Unable to compute mean: A task was canceled. 
//        
//       Cancelling at task 10 
//       Unable to compute mean: A task was canceled. 
//        
//       Calculating overall mean... 
//       The mean is 5.29545454545455. 
//        
//       Cancelling at task 4 
//       Unable to compute mean: A task was canceled. 
//        
//       Cancelling at task 5 
//       Unable to compute mean: A task was canceled. 
//        
//       Cancelling at task 6 
//       Unable to compute mean: A task was canceled. 
//        
//       Calculating overall mean... 
//       The mean is 4.97363636363636. 
//        
//       Cancelling at task 4 
//       Unable to compute mean: A task was canceled. 
//        
//       Cancelling at task 5 
//       Unable to compute mean: A task was canceled. 
//        
//       Cancelling at task 4 
//       Unable to compute mean: A task was canceled. 
//        
//       Calculating overall mean... 
//       The mean is 4.86545454545455.

.NET Framework

Supported in: 4.6, 4.5, 4

.NET Framework Client Profile

Supported in: 4

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library
Was this page helpful?
(1500 characters remaining)
Thank you for your feedback
Show:
© 2015 Microsoft