How to: Prevent a Child Task from Attaching to its Parent

.NET Framework 4.5

This document demonstrates how to prevent a child task from attaching to the parent task. Preventing a child task from attaching to its parent is useful when you call a component that is written by a third party and that also uses tasks. For example, a third-party component that uses the TaskCreationOptions.AttachedToParent option to create a Task or Task<TResult> object can cause problems in your code if it is long-running or throws an unhandled exception.

The following example compares the effects of using the default options to the effects of preventing a child task from attaching to the parent. The example creates a Task object that calls into a third-party library that also uses a Task object. The third-party library uses the AttachedToParent option to create the Task object. The application uses the TaskCreationOptions.DenyChildAttach option to create the parent task. This option instructs the runtime to remove the AttachedToParent specification in child tasks.

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.
*/

Because a parent task does not finish until all child tasks finish, a long-running child task can cause the overall application to perform poorly. In this example, when the application uses the default options to create the parent task, the child task must finish before the parent task finishes. When the application uses the TaskCreationOptions.DenyChildAttach option, the child is not attached to the parent. Therefore, the application can perform additional work after the parent task finishes and before it must wait for the child task to finish.

Copy the example code and paste it in a Visual Studio project, or paste it in a file that is named DenyChildAttach.cs (DenyChildAttach.vb for Visual Basic), and then run the following command in a Visual Studio Command Prompt window.

Visual C#

csc.exe DenyChildAttach.cs

Visual Basic

vbc.exe DenyChildAttach.vb

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