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.

Continuation tasks in .NET TPL: exception handling in task chains

We saw in previous posts on TPL how to chain tasks using the ContinueWhenAll and ContinueWhenAny methods. We also discussed strategies of exception handling in tasks. If you don’t know how to use these techniques make sure to read those posts first.

If you have multiple tasks chained together then special care should be taken when handling exceptions. There’s no automatic way to propagate an exception thrown by a task to the continuation task(s). However, the continuation task can always check the status of the antecedent and extract any exceptions. It can then process it in some way.

We start the first task:

Task antecedentTask = Task.Factory.StartNew(() =>
{
	Console.WriteLine("Mother of all tasks.");
});

The first continuation task throws an exception:

Task continuationTaskWithException = antecedentTask.ContinueWith(antecedent =>
{
	Console.WriteLine("Task that throws an exception.");
	throw new Exception();
});

The continuation of the continuation checks the status of the above task and re-throws the inner exception of its aggregate exception:

Task taskToRunAfterExceptionTask = continuationTaskWithException.ContinueWith(antecedent =>
{
	if (antecedent.Status == TaskStatus.Faulted)
	{
		throw antecedent.Exception.InnerException;
	}
	Console.WriteLine("Task that checks if previous task has faulted.");
});

We wait for the second continuation task to finish and handle the aggregate exception in some way:

try
{
	taskToRunAfterExceptionTask.Wait();
}
catch (AggregateException ex)
{
	ex.Handle(innerException =>
	{
		Console.WriteLine("Handled exception is of type: {0}", innerException.GetType());
		return true;
	});
}

If the second continuation task does not check the status of the task before that then the exception thrown by continuationTaskWithException will remain unhandled.

You can handle exceptions in a similar way when doing multitask continuations:

Task<int>[] motherTasks = new Task<int>[10];

Task<int> continuation = Task.Factory.ContinueWhenAll<int>(motherTasks, antecedents =>
{
	foreach (Task<int> task in antecedents)
	{
		if (task.Status == TaskStatus.Faulted)
		{
			//do something with the exception
		}
		else
		{
			//normal operations
		}
	}
	return 1234;
});

Handling exceptions in a ContinueWhenAny scenario is somewhat more complicated. Recall that even if a continuation task will have one antecedent – the one that finishes first – the other tasks in the antecedent task array will continue to run in the background. If one of those background tasks throws an exception then it will remain unhandled. A possible solution is to use ContinueWhenAll only for exception handling purposes:

Task<int>[] motherTasks = new Task<int>[10];

Task continuationTask = Task.Factory.ContinueWhenAny<int>(motherTasks,
	(Task<int> antecedent) =>
	{
		Console.WriteLine("Continuation task.");
	}, TaskContinuationOptions.NotOnFaulted);

Task exceptionHandlingContinuation = Task.Factory.ContinueWhenAll(motherTasks
	, antecedents =>
		{
			foreach (Task task in antecedents)
			{
				if (task.Status == TaskStatus.Faulted)
				{
					//exception handling code here
				}
			}
		});

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

Continuation tasks in .NET TPL: cancelling continuation tasks

Tasks in .NET TPL make it easy to assign tasks that should run upon the completion of a certain task.

We saw in previous posts on TPL how to define continuation tasks. You’ll also find examples showing how to cancel tasks in TPL. Continuation tasks can be cancelled using the same techniques as with “normal” tasks. If you don’t know the basics, then make sure to check out the related posts on this page.

We declare the cancellation token source:

CancellationTokenSource cancellationTokenSource	= new CancellationTokenSource();

We then continue with the antecedent task and provide the token source in the constructor:

Task motherTask = Task.Factory.StartNew(() =>
{
	Console.WriteLine("Mother task running.");
	cancellationTokenSource.Token.WaitHandle.WaitOne();
	cancellationTokenSource.Token.ThrowIfCancellationRequested();
}, cancellationTokenSource.Token);

We wait indefinitely for the token source to be cancelled with the WaitOne() method.

The following continuation task is created with the same cancellation token. This has the effect that if the token source is cancelled then both the antecedent and the continuation tasks will be cancelled:

Task taskSharingCancellationToken = motherTask.ContinueWith(antecedent =>
{
	Console.WriteLine("This continuation task will never run as it shares the cancellation token with the antecedent.");
}, cancellationTokenSource.Token);

If you remove the token from the constructor then this continuation task will run after the antecedent has completed with an exception.

The following continuation task may seem correct at first as it should only run if the antecedent has been cancelled:

Task incorrectContinuation = motherTask.ContinueWith(antecedent =>
{
       Console.WriteLine("This task will never be scheduled");
}, cancellationTokenSource.Token, TaskContinuationOptions.OnlyOnCanceled, TaskScheduler.Current);

However, it also shares the same cancellation token as the antecedent, so it will never run. In other words if you want a continuation task to run even if the antecedent has been cancelled then do not share the same cancellation token across tasks.

The below task will run as it doesn’t share the cancellation token:

Task correctContinuation = motherTask.ContinueWith(antecedent =>
{
	Console.WriteLine("Continuation running as there's no cancellation token sharing.");
}, TaskContinuationOptions.OnlyOnCanceled);

We cancel the token and wait for the correct continuation task to finish:

cancellationTokenSource.Cancel();
correctContinuation.Wait();

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

Continuation tasks in .NET TPL: any task in an array continued by a single task

Tasks in .NET TPL make it easy to assign tasks that should run upon the completion of a certain task.

In this post we saw how to create many tasks followed by a single continuation task that runs after all the antecedents have completed. We can also have the following scenario: a group of antecedent tasks are started and a single continuation task will be scheduled to run when the first task in the array of antecedent tasks has completed. You use the ContinueWhenAny() method to achieve this.

Make sure to read the post referred to above as ContinueWhenAll() and ContinueWhenAny() are very similar. Read especially the section on return values as it also applies to this post: just replace “ContinueWhenAll” with “ContinueWhenAny” in the examples. Also, read this post to apply a condition when the continuation task should be scheduled.

In the below example we’ll create a range of tasks that all return an integer: the number of milliseconds they were put to sleep. A CancellationToken object is used to introduce a random pause whose length is randomised between 100 and 1000 milliseconds. The access to the Random object is synchronised by a synchronisation primitive.

We declare a couple of elements first:

Task<int>[] motherTasks = new Task<int>[10];
CancellationTokenSource cancellationTokenSource = new CancellationTokenSource();

Random random = new Random();
Object syncPrimitive = new object();

The task array is filled and each task is started and returns an integer:

for (int i = 0; i < 10; i++)
{
	motherTasks[i] = Task.Factory.StartNew<int>(() =>
	{
		int taskSleepInterval;
		lock (syncPrimitive)
		{
			taskSleepInterval = random.Next(100, 1000);
		}
		cancellationTokenSource.Token.WaitHandle.WaitOne(taskSleepInterval);
		cancellationTokenSource.Token.ThrowIfCancellationRequested();
		return taskSleepInterval;
	}, cancellationTokenSource.Token);
}

Next we declare the continuation task:

Task continuationTask = Task.Factory.ContinueWhenAny<int>(motherTasks,
	(Task<int> antecedent) =>
	{
		Console.WriteLine("The first task slept for {0} milliseconds",
			antecedent.Result);
	});

The continuation task will be scheduled to run as soon as the first task in the antecedent task array has completed.

We wait for the continuation task to finish and then cancel the token source:

continuationTask.Wait();
cancellationTokenSource.Cancel();

Run the code and you’ll see that the continuation task reports the result from the first task.

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