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How to: Cancel a Task and Its Children

These examples show how to perform the following tasks:

  1. Create and start a cancelable task.

  2. Pass a cancellation token to your user delegate and optionally to the task instance.

  3. Notice and respond to the cancellation request in your user delegate.

  4. Optionally notice on the calling thread that the task was canceled.

The calling thread does not forcibly end the task; it only signals that cancellation is requested. If the task is already running, it is up to the user delegate to notice the request and respond appropriately. If cancellation is requested before the task runs, then the user delegate is never executed and the task object transitions into the Canceled state.

This example shows how to terminate a Task and its children in response to a cancellation request. It also shows that when a user delegate terminates by throwing a TaskCanceledException, the calling thread can optionally use the Wait method or WaitAll method to wait for the tasks to finish. In this case, you must use a try/catch block to handle the exceptions on the calling thread.

Imports System.Collections.Concurrent
Imports System.Threading
Imports System.Threading.Tasks

Module Example
    Sub Main()
        Dim tokenSource As New CancellationTokenSource()
        Dim token As CancellationToken = tokenSource.Token

        ' Store references to the tasks so that we can wait on them and   
        ' observe their status after cancellation.  
        Dim t As Task
        Dim tasks As New ConcurrentBag(Of Task)()

        Console.WriteLine("Press any key to begin tasks...")
        Console.WriteLine("To terminate the example, press 'c' to cancel and exit...")
        Console.ReadKey()
        Console.WriteLine()

        ' Request cancellation of a single task when the token source is canceled.  
        ' Pass the token to the user delegate, and also to the task so it can   
        ' handle the exception correctly.
        t = Task.Factory.StartNew(Sub() DoSomeWork(1, token), token)
        Console.WriteLine("Task {0} executing", t.Id)
        tasks.Add(t)

        ' Request cancellation of a task and its children. Note the token is passed  
        ' to (1) the user delegate and (2) as the second argument to StartNew, so   
        ' that the task instance can correctly handle the OperationCanceledException.
        t = Task.Factory.StartNew(Sub() 
                                     ' Create some cancelable child tasks.   
                                     Dim tc As Task
                                     For i As Integer = 3 To 10
                                        ' For each child task, pass the same token  
                                        ' to each user delegate and to StartNew.
                                        tc = Task.Factory.StartNew(Sub(iteration) DoSomeWork(iteration, token), i, token)
                                        Console.WriteLine("Task {0} executing", tc.Id)
                                        tasks.Add(tc)
                                        ' Pass the same token again to do work on the parent task.   
                                        ' All will be signaled by the call to tokenSource.Cancel below.
                                        DoSomeWork(2, token)
                                     Next  
                                  End Sub, 
                                  token)

        Console.WriteLine("Task {0} executing", t.Id)
        tasks.Add(t)

        ' Request cancellation from the UI thread.  
        If Console.ReadKey().KeyChar = "c"c Then
            tokenSource.Cancel()
            Console.WriteLine(vbCrLf + "Task cancellation requested.")

            ' Optional: Observe the change in the Status property on the task.  
            ' It is not necessary to wait on tasks that have canceled. However,  
            ' if you do wait, you must enclose the call in a try-catch block to  
            ' catch the TaskCanceledExceptions that are thrown. If you do   
            ' not wait, no exception is thrown if the token that was passed to the   
            ' StartNew method is the same token that requested the cancellation.  
        End If  

        Try
            Task.WaitAll(tasks.ToArray())
        Catch e As AggregateException
           Console.WriteLine()
           Console.WriteLine("AggregateException thrown with the following inner exceptions:")
           ' Display information about each exception.  
           For Each v In e.InnerExceptions
              If TypeOf v Is TaskCanceledException
                 Console.WriteLine("   TaskCanceledException: Task {0}", 
                                   DirectCast(v, TaskCanceledException).Task.Id)
              Else
                 Console.WriteLine("   Exception: {0}", v.GetType().Name)
              End If 
           Next 
           Console.WriteLine()
        End Try  

        ' Display status of all tasks.  
        For Each t In tasks
            Console.WriteLine("Task {0} status is now {1}", t.Id, t.Status)
        Next  
    End Sub  

    Sub DoSomeWork(ByVal taskNum As Integer, ByVal ct As CancellationToken)
        ' Was cancellation already requested?  
        If ct.IsCancellationRequested = True Then
            Console.WriteLine("Task {0} was cancelled before it got started.",
                              taskNum)
            ct.ThrowIfCancellationRequested()
        End If  

        Dim maxIterations As Integer = 100

        ' NOTE!!! A "TaskCanceledException was unhandled  
        ' by user code" error will be raised here if "Just My Code"  
        ' is enabled on your computer. On Express editions JMC is  
        ' enabled and cannot be disabled. The exception is benign.  
        ' Just press F5 to continue executing your code.  
        For i As Integer = 0 To maxIterations
            ' Do a bit of work. Not too much.  
            Dim sw As New SpinWait()
            For j As Integer = 0 To 100
                sw.SpinOnce()
            Next 
            If ct.IsCancellationRequested Then
                Console.WriteLine("Task {0} cancelled", taskNum)
                ct.ThrowIfCancellationRequested()
            End If  
        Next  
    End Sub  
End Module 
' The example displays output like the following: 
'    Press any key to begin tasks... 
'    To terminate the example, press 'c' to cancel and exit... 
'     
'    Task 1 executing 
'    Task 2 executing 
'    Task 3 executing 
'    Task 4 executing 
'    Task 5 executing 
'    Task 6 executing 
'    Task 7 executing 
'    Task 8 executing 
'    c 
'    Task cancellation requested. 
'    Task 2 cancelled 
'    Task 7 cancelled 
'     
'    AggregateException thrown with the following inner exceptions: 
'       TaskCanceledException: Task 2 
'       TaskCanceledException: Task 8 
'       TaskCanceledException: Task 7 
'     
'    Task 2 status is now Canceled 
'    Task 1 status is now RanToCompletion 
'    Task 8 status is now Canceled 
'    Task 7 status is now Canceled 
'    Task 6 status is now RanToCompletion 
'    Task 5 status is now RanToCompletion 
'    Task 4 status is now RanToCompletion 
'    Task 3 status is now RanToCompletion

The System.Threading.Tasks.Task class is fully integrated with the cancellation model that is based on the System.Threading.CancellationTokenSource and System.Threading.CancellationToken types. For more information, see Cancellation in Managed Threads and Task Cancellation.

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