CancellationTokenSource.Cancel Method

Definition

Namespace:

System.Threading

Assemblies:

netstandard.dll, System.Runtime.dll

Communicates a request for cancellation.

Overloads

Cancel()	Communicates a request for cancellation.
Cancel(Boolean)	Communicates a request for cancellation, and specifies whether remaining callbacks and cancelable operations should be processed if an exception occurs.

Cancel()

Source:

CancellationTokenSource.cs

Communicates a request for cancellation.

C#

public void Cancel();

Exceptions

ObjectDisposedException

This CancellationTokenSource has been disposed.

AggregateException

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

Examples

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.

C#

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.

Remarks

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.

We recommend that cancelable operations and callbacks registered with CancellationToken not throw exceptions.

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.

Calling this method has the same effect as calling Cancel(false).

See also

• Cancellation
• Task Cancellation

Cancel(Boolean)

Source:

CancellationTokenSource.cs

Communicates a request for cancellation, and specifies whether remaining callbacks and cancelable operations should be processed if an exception occurs.

C#

public void Cancel(bool throwOnFirstException);

Parameters

throwOnFirstException

Boolean

true if exceptions should immediately propagate; otherwise, false.

Exceptions

ObjectDisposedException

This CancellationTokenSource has been disposed.

AggregateException

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

Remarks

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. Callbacks will be executed synchronously in LIFO order.

We recommend that cancelable operations and callbacks registered with CancellationToken not throw exceptions.

If throwOnFirstException is true, an exception will immediately propagate out of the call to Cancel, preventing the remaining callbacks and cancelable operations from being processed.

If throwOnFirstException is false, this overload will aggregate any exceptions thrown into an AggregateException, such that one callback throwing an exception will not prevent other registered callbacks from being executed.

See also

• Cancellation
• Task Cancellation