Procedure: Een taak en de onderliggende items annuleren
In deze voorbeelden ziet u hoe u de volgende taken uitvoert:
Een annuleerbare taak maken en starten.
Geef een annuleringstoken door aan de gemachtigde van uw gebruiker en optioneel aan het taakexemplaren.
U ziet en reageert op de annuleringsaanvraag in de gemachtigde van de gebruiker.
U kunt eventueel op de aanroepende thread zien dat de taak is geannuleerd.
De aanroepende thread beëindigt de taak niet geforceerd; het geeft alleen aan dat annulering wordt aangevraagd. Als de taak al wordt uitgevoerd, is het aan de gemachtigde van de gebruiker om de aanvraag te zien en op de juiste manier te reageren. Als annulering wordt aangevraagd voordat de taak wordt uitgevoerd, wordt de gemachtigde van de gebruiker nooit uitgevoerd en wordt het taakobject overgezet naar de status Geannuleerd.
Opmerking
In dit voorbeeld ziet u hoe u een Task en de onderliggende items beëindigt als reactie op een annuleringsaanvraag. Het laat ook zien dat wanneer een gebruiker gedelegeerde wordt beëindigd door een TaskCanceledException, de aanroepende thread optioneel de Wait methode of WaitAll methode kan gebruiken om te wachten tot de taken zijn voltooid. In dit geval moet u een try/catch blok gebruiken om de uitzonderingen op de aanroepende thread af te handelen.
using System;
using System.Collections.Concurrent;
using System.Threading;
using System.Threading.Tasks;
public class Example
{
public static async Task Main()
{
var tokenSource = new CancellationTokenSource();
var token = tokenSource.Token;
// Store references to the tasks so that we can wait on them and
// observe their status after cancellation.
Task t;
var tasks = new ConcurrentBag<Task>();
Console.WriteLine("Press any key to begin tasks...");
Console.ReadKey(true);
Console.WriteLine("To terminate the example, press 'c' to cancel and exit...");
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.Run(() => DoSomeWork(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 Task.Run, so
// that the task instance can correctly handle the OperationCanceledException.
t = Task.Run(() =>
{
// Create some cancelable child tasks.
Task tc;
for (int i = 3; i <= 10; i++)
{
// For each child task, pass the same token
// to each user delegate and to Task.Run.
tc = Task.Run(() => DoSomeWork(token), 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(token);
}
}, token);
Console.WriteLine("Task {0} executing", t.Id);
tasks.Add(t);
// Request cancellation from the UI thread.
char ch = Console.ReadKey().KeyChar;
if (ch == 'c' || ch == 'C')
{
tokenSource.Cancel();
Console.WriteLine("\nTask 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 OperationCanceledExceptions that are thrown. If you do
// not wait, no exception is thrown if the token that was passed to the
// Task.Run method is the same token that requested the cancellation.
}
try
{
await Task.WhenAll(tasks.ToArray());
}
catch (OperationCanceledException)
{
Console.WriteLine($"\n{nameof(OperationCanceledException)} thrown\n");
}
finally
{
tokenSource.Dispose();
}
// Display status of all tasks.
foreach (var task in tasks)
Console.WriteLine("Task {0} status is now {1}", task.Id, task.Status);
}
static void DoSomeWork(CancellationToken ct)
{
// Was cancellation already requested?
if (ct.IsCancellationRequested)
{
Console.WriteLine("Task {0} was cancelled before it got started.",
Task.CurrentId);
ct.ThrowIfCancellationRequested();
}
int maxIterations = 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 (int i = 0; i <= maxIterations; i++)
{
// Do a bit of work. Not too much.
var sw = new SpinWait();
for (int j = 0; j <= 100; j++)
sw.SpinOnce();
if (ct.IsCancellationRequested)
{
Console.WriteLine("Task {0} cancelled", Task.CurrentId);
ct.ThrowIfCancellationRequested();
}
}
}
}
// 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
//
// OperationCanceledException thrown
//
// 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
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.ReadKey(True)
Console.WriteLine("To terminate the example, press 'c' to cancel and exit...")
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(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(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(token)
Next
End Sub,
token)
Console.WriteLine("Task {0} executing", t.Id)
tasks.Add(t)
' Request cancellation from the UI thread.
Dim ch As Char = Console.ReadKey().KeyChar
If ch = "c"c Or ch = "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 OperationCanceledExceptions 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 OperationCanceledException Then
Console.WriteLine(" The operation was canceled.")
Else
Console.WriteLine(" Exception: {0}", v.GetType().Name)
End If
Next
Console.WriteLine()
Finally
tokenSource.Dispose()
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 ct As CancellationToken)
' Was cancellation already requested?
If ct.IsCancellationRequested = True Then
Console.WriteLine("Task {0} was cancelled before it got started.",
Task.CurrentId)
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", Task.CurrentId)
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
De System.Threading.Tasks.Task klasse is volledig geïntegreerd met het annuleringsmodel dat is gebaseerd op de System.Threading.CancellationTokenSource en System.Threading.CancellationToken typen. Zie Annulering in Beheerde threads en Taakannulering voor meer informatie.
Zie ook
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