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CancellationToken Structure

Propagates notification that operations should be canceled.

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

'Declaration
Public Structure CancellationToken

The CancellationToken type exposes the following members.

  NameDescription
Public methodSupported by Portable Class LibraryCancellationTokenInitializes the CancellationToken.
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  NameDescription
Public propertySupported by Portable Class LibraryCanBeCanceledGets whether this token is capable of being in the canceled state.
Public propertySupported by Portable Class LibraryIsCancellationRequestedGets whether cancellation has been requested for this token.
Public propertyStatic memberSupported by Portable Class LibraryNoneReturns an empty CancellationToken value.
Public propertySupported by Portable Class LibraryWaitHandleGets a WaitHandle that is signaled when the token is canceled.
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  NameDescription
Public methodSupported by Portable Class LibraryEquals(CancellationToken)Determines whether the current CancellationToken instance is equal to the specified token.
Public methodSupported by Portable Class LibraryEquals(Object)Determines whether the current CancellationToken instance is equal to the specified Object. (Overrides ValueType.Equals(Object).)
Public methodSupported by Portable Class LibraryGetHashCodeServes as a hash function for a CancellationToken. (Overrides ValueType.GetHashCode.)
Public methodSupported by Portable Class LibraryGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodSupported by Portable Class LibraryRegister(Action)Registers a delegate that will be called when this CancellationToken is canceled.
Public methodSupported by Portable Class LibraryRegister(Action, Boolean)Registers a delegate that will be called when this CancellationToken is canceled.
Public methodSupported by Portable Class LibraryRegister(Action(Of Object), Object)Registers a delegate that will be called when this CancellationToken is canceled.
Public methodSupported by Portable Class LibraryRegister(Action(Of Object), Object, Boolean)Registers a delegate that will be called when this CancellationToken is canceled.
Public methodSupported by Portable Class LibraryThrowIfCancellationRequestedThrows a OperationCanceledException if this token has had cancellation requested.
Public methodSupported by Portable Class LibraryToStringReturns the fully qualified type name of this instance. (Inherited from ValueType.)
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  NameDescription
Public operatorStatic memberSupported by Portable Class LibraryEqualityDetermines whether two CancellationToken instances are equal.
Public operatorStatic memberSupported by Portable Class LibraryInequalityDetermines whether two CancellationToken instances are not equal.
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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(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

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library

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

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