Procédure : annuler une tâche et ses enfants
Ces exemples montrent comment effectuer les tâches suivantes :
Créer et lancer une tâche annulable
Passer un jeton d’annulation à votre délégué d’utilisateur et éventuellement à l’instance de tâche
Notifier et répondre à la demande d’annulation dans votre délégué d’utilisateur
Éventuellement notifier sur le thread appelant que la tâche a été annulée
Le thread appelant ne force pas l’arrêt de la tâche. Il signale seulement que son annulation a été demandée. Si la tâche est déjà en cours d’exécution, il incombe au délégué d’utilisateur de notifier la demande et d’y répondre de façon appropriée. Si l’annulation a été demandée avant l’exécution de la tâche, le délégué d’utilisateur n’est jamais exécuté et l’objet de tâche passe à l’état Canceled.
Exemple
Cet exemple montre comment arrêter une Task et ses enfants en réponse à une demande d’annulation. Il montre également que quand un délégué d’utilisateur se termine en levant une TaskCanceledException, le thread appelant peut éventuellement utiliser la méthode Wait ou WaitAllpour attendre la fin des tâches. Dans ce cas, vous devez utiliser un bloc try/catch pour gérer les exceptions sur le thread appelant.
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
La classe System.Threading.Tasks.Task est entièrement intégrée au modèle d’annulation qui est basé sur les types System.Threading.CancellationTokenSource et System.Threading.CancellationToken. Pour plus d’informations, consultez Annulation dans les threads managés et Annulation de tâches.
Voir aussi
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