CancellationTokenSource Class

Definition

Signals to a CancellationToken that it should be canceled.

public class CancellationTokenSource : IDisposable
[System.Runtime.InteropServices.ComVisible(false)]
public sealed class CancellationTokenSource : IDisposable
[System.Runtime.InteropServices.ComVisible(false)]
public class CancellationTokenSource : IDisposable
Inheritance
CancellationTokenSource
Attributes
Implements

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 completed 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.

Remarks

Starting with the .NET Framework 4, the .NET Framework uses a unified model for cooperative cancellation of asynchronous or long-running synchronous operations that involves two objects:

The general pattern for implementing the cooperative cancellation model is:

For more information, see Cancellation in Managed Threads.

Important

This type implements the IDisposable interface. When you have finished using an instance of the type, you should dispose of it either directly or indirectly. To dispose of the type directly, call its Dispose method in a try/finally block. To dispose of it indirectly, use a language construct such as using (in C#) or Using (in Visual Basic). For more information, see the "Using an Object that Implements IDisposable" section in the IDisposable interface topic.

Constructors

CancellationTokenSource()

Initializes a new instance of the CancellationTokenSource class.

CancellationTokenSource(Int32)

Initializes a new instance of the CancellationTokenSource class that will be canceled after the specified delay in milliseconds.

CancellationTokenSource(TimeSpan, TimeProvider)

Initializes a new instance of the CancellationTokenSource class that will be canceled after the specified TimeSpan.

CancellationTokenSource(TimeSpan)

Initializes a new instance of the CancellationTokenSource class that will be canceled after the specified time span.

Properties

IsCancellationRequested

Gets whether cancellation has been requested for this CancellationTokenSource.

Token

Gets the CancellationToken associated with this CancellationTokenSource.

Methods

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.

CancelAfter(Int32)

Schedules a cancel operation on this CancellationTokenSource after the specified number of milliseconds.

CancelAfter(TimeSpan)

Schedules a cancel operation on this CancellationTokenSource after the specified time span.

CancelAsync()

Communicates a request for cancellation asynchronously.

CreateLinkedTokenSource(CancellationToken, CancellationToken)

Creates a CancellationTokenSource that will be in the canceled state when any of the source tokens are in the canceled state.

CreateLinkedTokenSource(CancellationToken)

Creates a CancellationTokenSource that will be in the canceled state when the supplied token is in the canceled state.

CreateLinkedTokenSource(CancellationToken[])

Creates a CancellationTokenSource that will be in the canceled state when any of the source tokens in the specified array are in the canceled state.

CreateLinkedTokenSource(ReadOnlySpan<CancellationToken>)

Creates a CancellationTokenSource that will be in the canceled state when any of the source tokens are in the canceled state.

Dispose()

Releases all resources used by the current instance of the CancellationTokenSource class.

Dispose(Boolean)

Releases the unmanaged resources used by the CancellationTokenSource class and optionally releases the managed resources.

Equals(Object)

Determines whether the specified object is equal to the current object.

(Inherited from Object)
GetHashCode()

Serves as the default hash function.

(Inherited from Object)
GetType()

Gets the Type of the current instance.

(Inherited from Object)
MemberwiseClone()

Creates a shallow copy of the current Object.

(Inherited from Object)
ToString()

Returns a string that represents the current object.

(Inherited from Object)
TryReset()

Attempts to reset the CancellationTokenSource to be used for an unrelated operation.

Applies to

Product Versions
.NET Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard 1.0, 1.1, 1.2, 1.3, 1.4, 1.6, 2.0, 2.1
UWP 10.0

Thread Safety

All public and protected members of CancellationTokenSource are thread-safe and may be used concurrently from multiple threads, with the exception of Dispose(), which must only be used when all other operations on the CancellationTokenSource object have completed.

See also