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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 LibrarySupported in .NET for Windows Store appsCancellationTokenSourceInitializes a new instance of the CancellationTokenSource class.
Public methodSupported in .NET for Windows Store appsCancellationTokenSource(Int32)Initializes a new instance of the CancellationTokenSource class that will be canceled after the specified delay in milliseconds.
Public methodSupported in .NET for Windows Store appsCancellationTokenSource(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 LibrarySupported in .NET for Windows Store appsIsCancellationRequestedGets whether cancellation has been requested for this CancellationTokenSource.
Public propertySupported by Portable Class LibrarySupported in .NET for Windows Store appsTokenGets the CancellationToken associated with this CancellationTokenSource.
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  NameDescription
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsCancelCommunicates a request for cancellation.
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsCancel(Boolean)Communicates a request for cancellation, and specifies whether remaining callbacks and cancelable operations should be processed.
Public methodSupported in .NET for Windows Store appsCancelAfter(Int32)Schedules a cancel operation on this CancellationTokenSource after the specified number of milliseconds.
Public methodSupported in .NET for Windows Store appsCancelAfter(TimeSpan)Schedules a cancel operation on this CancellationTokenSource after the specified time span.
Public methodStatic memberSupported by Portable Class LibrarySupported in .NET for Windows Store appsCreateLinkedTokenSource(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 LibrarySupported in .NET for Windows Store appsCreateLinkedTokenSource(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 LibrarySupported in .NET for Windows Store appsDisposeReleases all resources used by the current instance of the CancellationTokenSource class.
Protected methodSupported in .NET for Windows Store appsDispose(Boolean)Releases the unmanaged resources used by the CancellationTokenSource class and optionally releases the managed resources.
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Protected methodSupported in .NET for Windows Store appsFinalizeAllows 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 LibrarySupported in .NET for Windows Store appsGetHashCodeServes as the default hash function. (Inherited from Object.)
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodSupported in .NET for Windows Store appsMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsToStringReturns a string that represents the current object. (Inherited from Object.)
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For more information, 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(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

.NET for Windows Store apps

Supported in: Windows 8

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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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