My continuation task appears to be complete before it finishes running

Question

I have a non-intuitive issue with a continuation task that I would think task.Wait would wait for the continuation task but it yields RanToCompletion before the task even finishes running? Here is the short source code. The output is below:

private static void TestChildTasks()
{
    Task t = Task.Run(() => RunParentTask());
    Task t2 = t.ContinueWith(task => Task.Run(() => RunChildTask()));
    //Task t2 = Task.Run(() => RunChildTask());
    Console.WriteLine("Waiting on t1");
    t.Wait();
    Console.WriteLine("Done waiting on t1");

    Console.WriteLine($"Waiting on t2, status of {t2.Status}");
    t2.Wait();

    Console.WriteLine($"Finished; child task is {t2.Status}");
}

private static void RunParentTask()
{
    Console.WriteLine("Parent Task is running");
    Thread.Sleep(2000);
    Console.WriteLine("Parent Task is done");
}

private static void RunChildTask()
{
    Console.WriteLine("Child task is running");
    Thread.Sleep(3000);
    Console.WriteLine("Child Task is done");
}

Here is the output:

Waiting on t1

Parent Task is running

Parent Task is done

Done waiting on t1

Waiting on t2, status of Running

Finished; child task is RanToCompletion

press enter to exit

Child task is running

Child Task is done

Why does the child task continue to run after it returns a status of RanToCompletion?

Answer 1

Lets look at the following line:

Task t2 = t.ContinueWith(task => Task.Run(() => RunChildTask()));

Even though you have declared t2 as Task it is really a Task<Task>:

Task<Task> t2 = t.ContinueWith(task => Task.Run(() => RunChildTask()));

Why? Because Task.Run creates a new Task.

You have several options to fix this:

Option #1 - Unwrap

To make a Task from a Task<Task> you need to call the Unwrap method.

Task t2 = t.ContinueWith(task => Task.Run(() => RunChildTask())).Unwrap();

With this modification the output will look like this:

Waiting on t1
Parent Task is running
Parent Task is done
Done waiting on t1
Child task is running
Waiting on t2, status of WaitingForActivation
Child Task is done
Finished; child task is RanToCompletion

Option #2 - Avoid new task creation

Actually you can call the RunChildTask without the Task.Run:

Task t2 = t.ContinueWith(_ => RunChildTask());

With this modification the output will look like this:

Waiting on t1
Parent Task is running
Parent Task is done
Done waiting on t1
Child task is running
Waiting on t2, status of Running
Child Task is done
Finished; child task is RanToCompletion

Option #3 - AttachToParent

If you want to you can attach a Task not just with the ContinueWith, but also with the Task.Factory.StartNew:

Task t2 = null;
Task t = Task.Factory.StartNew(() =>
{
    RunParentTask();
    t2 = Task.Factory.StartNew(RunChildTask, TaskCreationOptions.AttachedToParent);
});

With this modification the output will look like this:

Waiting on t1
Parent Task is running
Parent Task is done
Child task is running
Child Task is done
Done waiting on t1
Waiting on t2, status of RanToCompletion
Finished; child task is RanToCompletion

There are several other ways as well.