Como: Cancelar uma tarefa e seus filhos

  • Artigo

Estes exemplos mostram como realizar as seguintes tarefas:

  1. Crie e inicie uma tarefa cancelável.

  2. Passe um token de cancelamento para o representante de usuário e, opcionalmente, para a instância da tarefa.

  3. Observe e responda à solicitação de cancelamento de seu representante de usuário.

  4. 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()
    {
        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

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