Creating an attached child Task in .NET C#

A child Task, a.k.a a nested Task, is a Task that’s started within the body of another Task. The containing Task is called the parent.

We saw here how to create detached child Tasks. They are not very interesting really. An attached child Task however is one which has a special relationship with the parent:

  • The parent waits for the child to finish before it completes
  • If the child Task throws an exception then the parent will catch it and re-throw it
  • The status of the parent depends on the status of the child

This is how you attach a child task to a parent:

Task parentTask = Task.Factory.StartNew(() =>
{	
        Console.WriteLine("Starting child task...");			
	Task childTask = Task.Factory.StartNew(() =>
	{
		Console.WriteLine("Child running. Going to sleep for a sec.");
		Thread.Sleep(1000);
		Console.WriteLine("Child finished and throws an exception.");
		throw new Exception();
	}, TaskCreationOptions.AttachedToParent);				
});

try
{
	Console.WriteLine("Waiting for parent task");
	parentTask.Wait();
	Console.WriteLine("Parent task finished");
}
catch (AggregateException ex)
{
	Console.WriteLine("Exception: {0}", ex.InnerException.GetType());
}

Note the TaskCreationOptions parameter in the StartNew method: AttachedToParent. If you run this code the you’ll see that the exception thrown by the child task is caught in the try-catch block. The original exception has been packaged within an AggregateException. Also, the Wait() method will wait for the parent task to finish which in turn waits for the child to finish. So Wait() indirectly waits for any attached children the parent Task may have. Note also that as the parent task finishes, its status is updated to ‘WaitingForChildrenToComplete’ which means exactly what it implies.

View the list of posts on the Task Parallel Library here.

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About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

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