CancellationToken Struct

Definition

public value class CancellationToken

public value class CancellationToken : IEquatable<System::Threading::CancellationToken>

public struct CancellationToken

public readonly struct CancellationToken : IEquatable<System.Threading.CancellationToken>

public readonly struct CancellationToken

[System.Runtime.InteropServices.ComVisible(false)]
public struct CancellationToken

type CancellationToken = struct

[<System.Runtime.InteropServices.ComVisible(false)>]
type CancellationToken = struct

Public Structure CancellationToken

Public Structure CancellationToken
Implements IEquatable(Of CancellationToken)

Inheritance

Object

ValueType

CancellationToken

Attributes

ComVisibleAttribute

Implements

IEquatable<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 that's passed to a TaskFactory object. In turn, the TaskFactory object 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 was cancelled, 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.

Imports System.Collections.Generic
Imports System.Threading
Imports System.Threading.Tasks

Module Example
 Public Sub Main()
 ' Define the cancellation token.
 Dim source As New CancellationTokenSource()
 Dim token As CancellationToken = source.Token

 Dim lockObj As New Object()
 Dim rnd As New Random

 Dim tasks As New List(Of Task(Of Integer()))
 Dim factory As New TaskFactory(token)
 For taskCtr As Integer = 0 To 10
 Dim iteration As Integer = taskCtr + 1
 tasks.Add(factory.StartNew(Function()
 Dim value, values(9) As Integer
 For ctr As Integer = 1 To 10
 SyncLock lockObj
 value = rnd.Next(0,101)
 End SyncLock
 If value = 0 Then 
 source.Cancel
 Console.WriteLine("Cancelling at task {0}", iteration)
 Exit For
 End If 
 values(ctr-1) = value 
 Next
 Return values
 End Function, token)) 
 
 Next
 Try
 Dim fTask As Task(Of Double) = factory.ContinueWhenAll(tasks.ToArray(), 
 Function(results)
 Console.WriteLine("Calculating overall mean...")
 Dim sum As Long
 Dim n As Integer 
 For Each t In results
 For Each r In t.Result
 sum += r
 n+= 1
 Next
 Next
 Return sum/n
 End Function, token)
 Console.WriteLine("The mean is {0}.", fTask.Result)
 Catch ae As AggregateException
 For Each e In ae.InnerExceptions
 If TypeOf e Is TaskCanceledException
 Console.WriteLine("Unable to compute mean: {0}", 
 CType(e, TaskCanceledException).Message)
 Else
 Console.WriteLine("Exception: " + e.GetType().Name)
 End If 
 Next
 Finally
 source.Dispose()
 End Try 
 End Sub
End Module
' 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

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.

Thread Safety

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

See also

• Cancellation
• Task Cancellation