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Как предотвратить привязку дочернего задания к родительской задаче

  • Статья

В этом документе показано, как предотвратить присоединение дочерней задачи к родительской задаче. Предотвращение присоединения дочерней задачи к родительской задаче полезно при вызове компонента, написанного сторонним поставщиком, который также использует задачи. Например, сторонний компонент, использующий параметр TaskCreationOptions.AttachedToParent для создания Task или Task<TResult> объекта, может вызвать проблемы в коде, если он является длительным или вызывает необработанное исключение.

Пример

В следующем примере сравниваются эффекты использования параметров по умолчанию с эффектами препятствования присоединению дочерней задачи к родительской задаче. В этом примере создается объект Task, который вызывается в стороннюю библиотеку, которая также использует объект Task. В сторонней библиотеке используется параметр AttachedToParent для создания объекта Task. Приложение использует параметр TaskCreationOptions.DenyChildAttach для создания родительской задачи. Этот параметр указывает среде выполнения удалить спецификацию AttachedToParent в дочерних задачах.

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 {stopwatch.ElapsedMilliseconds} ms.");

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

      // 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 {stopwatch.ElapsedMilliseconds} ms.");
   }

   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.
'

Так как родительская задача не завершается до завершения всех дочерних задач, долго выполняющаяся дочерняя задача может привести к плохому выполнению общего приложения. В этом примере, когда приложение использует параметры по умолчанию для создания родительской задачи, дочерние задачи должны завершиться до завершения родительской задачи. Если приложение использует параметр TaskCreationOptions.DenyChildAttach, дочерний элемент не подключен к родительскому элементу. Таким образом, приложение может выполнить дополнительную работу после завершения родительской задачи и до завершения дочерней задачи.

См. также