Como: Cancelar uma tarefa e seus filhos
Este exemplo mostra como executar as seguintes tarefas:
- Crie e inicie uma tarefa cancelável.
- Passe um token de cancelamento para o representante de usuário e, opcionalmente, para a instância da tarefa.
- Observe e responda à solicitação de cancelamento de seu representante de usuário.
- Opcionalmente, observe no thread de chamada que a tarefa foi cancelada.
O thread de chamada não força o término da tarefa. Ele apenas sinaliza que o cancelamento é solicitado. Se a tarefa já estiver em execução, cabe ao representante de usuário observar a solicitação e responder adequadamente. Se cancelamento for solicitado antes da execução da tarefa, o representante de usuário nunca será executado e o objeto de tarefa fará a transição para o estado Cancelado.
Exemplo
Este exemplo mostra como terminar uma Task e suas tarefas filho em resposta a uma solicitação de cancelamento. Ele também mostra que quando um representante de usuário termina lançando um TaskCanceledException, o thread de chamada pode optar por usar o método Wait ou o método WaitAll para aguardar a conclusão das tarefas. Nesse caso, você deve usar um bloco try/catch para lidar com as exceções no thread de chamada.
using System;
using System.Collections.Concurrent;
using System.Threading;
using System.Threading.Tasks;
public class Example
{
public static async Task Main()
{
// Cancellation token source for cancellation. Make sure to dispose after use (which is done here through the using expression).
using var tokenSource = new CancellationTokenSource();
// The cancellation token will be used to communicate cancellation to tasks
var token = tokenSource.Token;
Console.WriteLine("Main: Press any key to begin tasks...");
Console.ReadKey(true);
Console.WriteLine("Main: To terminate the example, press 'c' to cancel and exit...");
Console.WriteLine();
// Store references to the tasks so that we can wait on them and
// observe their status after cancellation.
var tasks = new ConcurrentBag<Task>();
// Pass the token to the user delegate so it can cancel during execution,
// and also to the task so it can cancel before execution starts.
var cancellableTask = Task.Run(() => {
DoSomeWork(token);
Console.WriteLine("Cancellable: Task {0} ran to completion", Task.CurrentId);
}, token);
Console.WriteLine("Main: Cancellable Task {0} created", cancellableTask.Id);
tasks.Add(cancellableTask);
var parentTask = Task.Run(() =>
{
for (int i = 0; i <= 7; i++)
{
// If cancellation was requested we don't need to start any more
// child tasks (that would immediately cancel) => break out of loop
if (token.IsCancellationRequested) break;
// For each child task, pass the same token
// to each user delegate and to Task.Run.
var childTask = Task.Run(() => {
DoSomeWork(token);
Console.WriteLine("Child: Task {0} ran to completion", Task.CurrentId);
}, token);
Console.WriteLine("Parent: Task {0} created", childTask.Id);
tasks.Add(childTask);
DoSomeWork(token, maxIterations: 1);
}
Console.WriteLine("Parent: Task {0} ran to completion", Task.CurrentId);
}, token);
Console.WriteLine("Main: Parent Task {0} created", parentTask.Id);
tasks.Add(parentTask);
// Request cancellation from the UI thread.
char ch = Console.ReadKey().KeyChar;
if (ch == 'c' || ch == 'C')
{
tokenSource.Cancel();
Console.WriteLine("\nMain: 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
// Task.Run method is the same token that requested the cancellation.
}
try
{
// Wait for all tasks before disposing the cancellation token source
await Task.WhenAll(tasks);
}
catch (OperationCanceledException)
{
Console.WriteLine($"\nMain: {nameof(OperationCanceledException)} thrown\n");
}
// Display status of all tasks.
foreach (var task in tasks)
{
Console.WriteLine("Main: Task {0} status is now {1}", task.Id, task.Status);
}
}
static void DoSomeWork(CancellationToken ct, int maxIterations = 10)
{
// Was cancellation already requested?
if (ct.IsCancellationRequested)
{
Console.WriteLine("Task {0} was cancelled before it got started.", Task.CurrentId);
ct.ThrowIfCancellationRequested();
}
// 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} work cancelled", Task.CurrentId);
ct.ThrowIfCancellationRequested();
}
}
}
}
// The example displays output like the following:
// Main: Press any key to begin tasks...
// Main: To terminate the example, press 'c' to cancel and exit...
//
// Main: Cancellable Task 13 created
// Main: Parent Task 14 created
// Parent: Task 15 created
// Parent: Task 16 created
// Parent: Task 17 created
// Parent: Task 18 created
// Parent: Task 19 created
// Parent: Task 20 created
// Cancellable: Task 13 ran to completion
// Child: Task 15 ran to completion
// Parent: Task 21 created
// Child: Task 16 ran to completion
// Parent: Task 22 created
// Child: Task 17 ran to completion
// c
// Main: Task cancellation requested.
// Task 20 work cancelled
// Task 21 work cancelled
// Task 22 work cancelled
// Task 18 work cancelled
// Task 14 work cancelled
// Task 19 work cancelled
//
// Main: OperationCanceledException thrown
//
// Main: Task 22 status is now Canceled
// Main: Task 21 status is now Canceled
// Main: Task 20 status is now Canceled
// Main: Task 19 status is now Canceled
// Main: Task 18 status is now Canceled
// Main: Task 17 status is now RanToCompletion
// Main: Task 16 status is now RanToCompletion
// Main: Task 15 status is now RanToCompletion
// Main: Task 14 status is now Canceled
// Main: Task 13 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
A classe System.Threading.Tasks.Task está totalmente integrada com o modelo de cancelamento que se baseia nos tipos System.Threading.CancellationTokenSource e System.Threading.CancellationToken. Para saber mais, confira Cancelamento em threads gerenciados e Cancelamento de tarefas.
Confira também
Colaborar conosco no GitHub
A fonte deste conteúdo pode ser encontrada no GitHub, onde você também pode criar e revisar problemas e solicitações de pull. Para obter mais informações, confira o nosso guia para colaboradores.
