CancellationToken Structure
TOC
Collapse the table of content
Expand the table of content

CancellationToken Structure

.NET Framework (current version)
 

Propagates notification that operations should be canceled.

Namespace:   System.Threading
Assembly:  mscorlib (in mscorlib.dll)

[ComVisibleAttribute(false)]
[HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization = true, 
	ExternalThreading = true)]
public struct CancellationToken

NameDescription
System_CAPS_pubmethodCancellationToken(Boolean)

Initializes the CancellationToken.

NameDescription
System_CAPS_pubpropertyCanBeCanceled

Gets whether this token is capable of being in the canceled state.

System_CAPS_pubpropertyIsCancellationRequested

Gets whether cancellation has been requested for this token.

System_CAPS_pubpropertySystem_CAPS_staticNone

Returns an empty CancellationToken value.

System_CAPS_pubpropertyWaitHandle

Gets a WaitHandle that is signaled when the token is canceled.

NameDescription
System_CAPS_pubmethodEquals(CancellationToken)

Determines whether the current CancellationToken instance is equal to the specified token.

System_CAPS_pubmethodEquals(Object)

Determines whether the current CancellationToken instance is equal to the specified Object.(Overrides ValueType.Equals(Object).)

System_CAPS_pubmethodGetHashCode()

Serves as a hash function for a CancellationToken.(Overrides ValueType.GetHashCode().)

System_CAPS_pubmethodGetType()

Gets the Type of the current instance.(Inherited from Object.)

System_CAPS_pubmethodRegister(Action)

Registers a delegate that will be called when this CancellationToken is canceled.

System_CAPS_pubmethodRegister(Action, Boolean)

Registers a delegate that will be called when this CancellationToken is canceled.

System_CAPS_pubmethodRegister(Action<Object>, Object)

Registers a delegate that will be called when this CancellationToken is canceled.

System_CAPS_pubmethodRegister(Action<Object>, Object, Boolean)

Registers a delegate that will be called when this CancellationToken is canceled.

System_CAPS_pubmethodThrowIfCancellationRequested()

Throws a OperationCanceledException if this token has had cancellation requested.

System_CAPS_pubmethodToString()

Returns the fully qualified type name of this instance.(Inherited from ValueType.)

NameDescription
System_CAPS_puboperatorSystem_CAPS_staticEquality(CancellationToken, CancellationToken)

Determines whether two CancellationToken instances are equal.

System_CAPS_puboperatorSystem_CAPS_staticInequality(CancellationToken, CancellationToken)

Determines whether two CancellationToken instances are not equal.

A CancellationToken enables cooperative cancellation between threads, thread pool work items, or Task objects. You create a cancellation token by instantiating a CancellationTokenSource object, which manages cancellation tokens retrieved from its CancellationTokenSource.Token property. You then pass the cancellation token to any number of threads, tasks, or operations that should receive notice of cancellation. The token cannot be used to initiate cancellation. When the owning object calls CancellationTokenSource.Cancel, the IsCancellationRequested property on every copy of the cancellation token is set to true. The objects that receive the notification can respond in whatever manner is appropriate.

For more information and code examples see Cancellation in Managed Threads.

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<TAntecedentResult, TResult> 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.

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 5.0
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1

All public and protected members of CancellationToken are thread-safe and may be used concurrently from multiple threads.

Return to top
Show:
© 2016 Microsoft