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Como: evitar que uma tarefa filha se anexe ao pai dela

  • Artigo

Este documento demonstra como evitar que uma tarefa filho se anexe à tarefa principal. Impedir que uma tarefa filho seja anexada à tarefa pai é útil quando você chama um componente que é gravado por um terceiro e que também usa tarefas. Por exemplo, um componente de terceiros que usa a opção TaskCreationOptions.AttachedToParent para criar um objeto Task ou Task<TResult> pode causar problemas em seu código se ele for de longa duração ou lançar uma exceção não controlada.

Exemplo

O exemplo a seguir compara os efeitos de usar as opções padrão com os efeitos de impedir que uma tarefa filho seja anexada ao pai. O exemplo cria um objeto Task que chama para uma biblioteca de terceiros que também usa um objeto Task. A biblioteca de terceiros usa a opção AttachedToParent para criar o objeto Task. O aplicativo usa a opção TaskCreationOptions.DenyChildAttach para criar a tarefa pai. Essa opção instrui o runtime para remover a especificação AttachedToParent em tarefas filho.

using System;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;

// Defines functionality that is provided by a third-party.
// In a real-world scenario, this would likely be provided
// in a separate code file or assembly.
namespace Contoso
{
   public class Widget
   {
      public Task Run()
      {
         // Create a long-running task that is attached to the
         // parent in the task hierarchy.
         return Task.Factory.StartNew(() =>
         {
            // Simulate a lengthy operation.
            Thread.Sleep(5000);
         }, TaskCreationOptions.AttachedToParent);
      }
   }
}

// Demonstrates how to prevent a child task from attaching to the parent.
class DenyChildAttach
{
   static void RunWidget(Contoso.Widget widget,
      TaskCreationOptions parentTaskOptions)
   {
      // Record the time required to run the parent
      // and child tasks.
      Stopwatch stopwatch = new Stopwatch();
      stopwatch.Start();

      Console.WriteLine("Starting widget as a background task...");

      // Run the widget task in the background.
      Task<Task> runWidget = Task.Factory.StartNew(() =>
         {
            Task widgetTask = widget.Run();

            // Perform other work while the task runs...
            Thread.Sleep(1000);

            return widgetTask;
         }, parentTaskOptions);

      // Wait for the parent task to finish.
      Console.WriteLine("Waiting for parent task to finish...");
      runWidget.Wait();
      Console.WriteLine("Parent task has finished. Elapsed time is {0} ms.",
         stopwatch.ElapsedMilliseconds);

      // Perform more work...
      Console.WriteLine("Performing more work on the main thread...");
      Thread.Sleep(2000);
      Console.WriteLine("Elapsed time is {0} ms.", stopwatch.ElapsedMilliseconds);

      // Wait for the child task to finish.
      Console.WriteLine("Waiting for child task to finish...");
      runWidget.Result.Wait();
      Console.WriteLine("Child task has finished. Elapsed time is {0} ms.",
        stopwatch.ElapsedMilliseconds);
   }

   static void Main(string[] args)
   {
      Contoso.Widget w = new Contoso.Widget();

      // Perform the same operation two times. The first time, the operation
      // is performed by using the default task creation options. The second
      // time, the operation is performed by using the DenyChildAttach option
      // in the parent task.

      Console.WriteLine("Demonstrating parent/child tasks with default options...");
      RunWidget(w, TaskCreationOptions.None);

      Console.WriteLine();

      Console.WriteLine("Demonstrating parent/child tasks with the DenyChildAttach option...");
      RunWidget(w, TaskCreationOptions.DenyChildAttach);
   }
}

/* Sample output:
Demonstrating parent/child tasks with default options...
Starting widget as a background task...
Waiting for parent task to finish...
Parent task has finished. Elapsed time is 5014 ms.
Performing more work on the main thread...
Elapsed time is 7019 ms.
Waiting for child task to finish...
Child task has finished. Elapsed time is 7019 ms.

Demonstrating parent/child tasks with the DenyChildAttach option...
Starting widget as a background task...
Waiting for parent task to finish...
Parent task has finished. Elapsed time is 1007 ms.
Performing more work on the main thread...
Elapsed time is 3015 ms.
Waiting for child task to finish...
Child task has finished. Elapsed time is 5015 ms.
*/

Imports System.Diagnostics
Imports System.Threading
Imports System.Threading.Tasks

' Defines functionality that is provided by a third-party.
' In a real-world scenario, this would likely be provided
' in a separate code file or assembly.
Namespace Contoso
    Public Class Widget
        Public Function Run() As Task
            ' Create a long-running task that is attached to the 
            ' parent in the task hierarchy.
            Return Task.Factory.StartNew(Sub() Thread.Sleep(5000), TaskCreationOptions.AttachedToParent)
            ' Simulate a lengthy operation.
        End Function
    End Class
End Namespace

' Demonstrates how to prevent a child task from attaching to the parent.
Friend Class DenyChildAttach
    Private Shared Sub RunWidget(ByVal widget As Contoso.Widget, ByVal parentTaskOptions As TaskCreationOptions)
        ' Record the time required to run the parent
        ' and child tasks.
        Dim stopwatch As New Stopwatch()
        stopwatch.Start()

        Console.WriteLine("Starting widget as a background task...")

        ' Run the widget task in the background.
        Dim runWidget As Task(Of Task) = Task.Factory.StartNew(Function()
                                                                   ' Perform other work while the task runs...
                                                                   Dim widgetTask As Task = widget.Run()
                                                                   Thread.Sleep(1000)
                                                                   Return widgetTask
                                                               End Function, parentTaskOptions)

        ' Wait for the parent task to finish.
        Console.WriteLine("Waiting for parent task to finish...")
        runWidget.Wait()
        Console.WriteLine("Parent task has finished. Elapsed time is {0} ms.", stopwatch.ElapsedMilliseconds)

        ' Perform more work...
        Console.WriteLine("Performing more work on the main thread...")
        Thread.Sleep(2000)
        Console.WriteLine("Elapsed time is {0} ms.", stopwatch.ElapsedMilliseconds)

        ' Wait for the child task to finish.
        Console.WriteLine("Waiting for child task to finish...")
        runWidget.Result.Wait()
        Console.WriteLine("Child task has finished. Elapsed time is {0} ms.", stopwatch.ElapsedMilliseconds)
    End Sub

    Shared Sub Main(ByVal args() As String)
        Dim w As New Contoso.Widget()

        ' Perform the same operation two times. The first time, the operation
        ' is performed by using the default task creation options. The second
        ' time, the operation is performed by using the DenyChildAttach option
        ' in the parent task.

        Console.WriteLine("Demonstrating parent/child tasks with default options...")
        RunWidget(w, TaskCreationOptions.None)

        Console.WriteLine()

        Console.WriteLine("Demonstrating parent/child tasks with the DenyChildAttach option...")
        RunWidget(w, TaskCreationOptions.DenyChildAttach)
    End Sub
End Class

' Sample output:
'Demonstrating parent/child tasks with default options...
'Starting widget as a background task...
'Waiting for parent task to finish...
'Parent task has finished. Elapsed time is 5014 ms.
'Performing more work on the main thread...
'Elapsed time is 7019 ms.
'Waiting for child task to finish...
'Child task has finished. Elapsed time is 7019 ms.
'
'Demonstrating parent/child tasks with the DenyChildAttach option...
'Starting widget as a background task...
'Waiting for parent task to finish...
'Parent task has finished. Elapsed time is 1007 ms.
'Performing more work on the main thread...
'Elapsed time is 3015 ms.
'Waiting for child task to finish...
'Child task has finished. Elapsed time is 5015 ms.
'

Como a tarefa pai não termina até que todas as tarefas filho sejam concluídas, uma tarefa filho de longa duração pode fazer com que o aplicativo geral tenha um baixo desempenho. Neste exemplo, quando o aplicativo usa as opções padrão para criar a tarefa pai, a tarefa filho deve terminar antes da conclusão da tarefa pai. Quando o aplicativo usa a opção TaskCreationOptions.DenyChildAttach, a tarefa filho não está anexada à tarefa pai. Portanto, o aplicativo pode executar trabalhos adicionais após a conclusão da tarefa pai e antes de aguardar a conclusão da tarefa filho.

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