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CancellationTokenSource Class

Signals to a CancellationToken that it should be canceled.

System.Object
  System.Threading.CancellationTokenSource

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

'Declaration
Public Class CancellationTokenSource _
	Implements IDisposable

The CancellationTokenSource type exposes the following members.

  NameDescription
Public methodSupported by Portable Class LibraryCancellationTokenSourceInitializes a new instance of the CancellationTokenSource class.
Public methodCancellationTokenSource(Int32)Initializes a new instance of the CancellationTokenSource class that will be canceled after the specified delay in milliseconds.
Public methodCancellationTokenSource(TimeSpan)Initializes a new instance of the CancellationTokenSource class that will be canceled after the specified time span.
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  NameDescription
Public propertySupported by Portable Class LibraryIsCancellationRequestedGets whether cancellation has been requested for this CancellationTokenSource.
Public propertySupported by Portable Class LibraryTokenGets the CancellationToken associated with this CancellationTokenSource.
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  NameDescription
Public methodSupported by Portable Class LibraryCancelCommunicates a request for cancellation.
Public methodSupported by Portable Class LibraryCancel(Boolean)Communicates a request for cancellation, and specifies whether remaining callbacks and cancelable operations should be processed.
Public methodCancelAfter(Int32)Schedules a cancel operation on this CancellationTokenSource after the specified number of milliseconds.
Public methodCancelAfter(TimeSpan)Schedules a cancel operation on this CancellationTokenSource after the specified time span.
Public methodStatic memberSupported by Portable Class LibraryCreateLinkedTokenSource(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.
Public methodStatic memberSupported by Portable Class LibraryCreateLinkedTokenSource(CancellationToken, CancellationToken)Creates a CancellationTokenSource that will be in the canceled state when any of the source tokens are in the canceled state.
Public methodSupported by Portable Class LibraryDisposeReleases all resources used by the current instance of the CancellationTokenSource class.
Protected methodDispose(Boolean)Releases the unmanaged resources used by the CancellationTokenSource class and optionally releases the managed resources.
Public methodSupported by Portable Class LibraryEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Protected methodFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodSupported by Portable Class LibraryGetHashCodeServes as the default hash function. (Inherited from Object.)
Public methodSupported by Portable Class LibraryGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodSupported by Portable Class LibraryToStringReturns a string that represents the current object. (Inherited from Object.)
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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:

  • A CancellationTokenSource object, which provides a cancellation token through its Token property and sends a cancellation message by calling its Cancel or CancelAfter method.

  • A CancellationTokenSource object, which indicates whether cancellation is requested.

The general pattern for implementing the cooperative cancellation model is:

  • Instantiate a CancellationTokenSource object, which manages and sends cancellation notification to the individual cancellation tokens.

  • Pass the token returned by the CancellationTokenSource.Token property to each task or thread that listens for cancellation.

  • Call the CancellationToken.IsCancellationRequested method from operations that receive the cancellation token. Provide a mechanism for each task or thread to respond to a cancellation request. Whether you choose to cancel an operation, and exactly how you do it, depends on your application logic.

  • Call the CancellationTokenSource.Cancel method to provide notification of cancellation. This sets the CancellationToken.IsCancellationRequested property on every copy of the cancellation token to true.

  • Call the Dispose method when you are finished with the CancellationTokenSource object.

For more information, see Cancellation in Managed Threads.

Important noteImportant

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/catch 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.

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(Of TAntecedentResult, TResult)(Task(Of TAntecedentResult)(), Func(Of Task(Of 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.

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.

.NET Framework

Supported in: 4.6, 4.5, 4

.NET Framework Client Profile

Supported in: 4

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

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.

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