Prallelisation Talk Example – Tasks within Tasks

In this example I’m showing the launching of further tasks within an existing task.

The Main method launches a single task (of course, it would likely be many tasks in a real system) which is implemented by MainTask and then waits for that task to complete. The MainTask then launches many independent tasks (impelemnted as SubTask and attaches each of them to the parent task (in this case MainTask). This has the effect that when MainTask ends the code in Main is still blocked at the Wait call until all the child tasks have also completed.

Code Example

class Program
{
    static void Main(string[] args)
    {
        Task t = Task.Factory.StartNew(MainTask);
        t.Wait();

        Console.WriteLine("Program finished");
        Console.ReadLine();
    }

    public static void MainTask()
    {
        Console.WriteLine("Starting the Main Task");

        Task.Factory.StartNew(SubTask, TaskCreationOptions.AttachedToParent);
        Task.Factory.StartNew(SubTask, TaskCreationOptions.AttachedToParent);
        Task.Factory.StartNew(SubTask, TaskCreationOptions.AttachedToParent);
        Task.Factory.StartNew(SubTask, TaskCreationOptions.AttachedToParent);
        Task.Factory.StartNew(SubTask, TaskCreationOptions.AttachedToParent);
        Task.Factory.StartNew(SubTask, TaskCreationOptions.AttachedToParent);
        Task.Factory.StartNew(SubTask, TaskCreationOptions.AttachedToParent);
        Task.Factory.StartNew(SubTask, TaskCreationOptions.AttachedToParent);
        Task.Factory.StartNew(SubTask, TaskCreationOptions.AttachedToParent);
        Task.Factory.StartNew(SubTask, TaskCreationOptions.AttachedToParent);

        Console.WriteLine("Ending the Main Task");
    }

    public static void SubTask()
    {
        Console.WriteLine("Starting SubTask {0}", Task.CurrentId);
        Thread.Sleep(2500);
        Console.WriteLine("Ending SubTask {0}", Task.CurrentId);
    }
}

Output

Starting the Main Task
Ending the Main Task
Starting SubTask 1
Starting SubTask 2
Starting SubTask 3
Starting SubTask 4
Starting SubTask 5
Starting SubTask 6
Ending SubTask 2
Starting SubTask 7
Ending SubTask 1
Starting SubTask 8
Ending SubTask 3
Starting SubTask 9
Ending SubTask 4
Starting SubTask 10
Ending SubTask 5
Ending SubTask 6
Ending SubTask 7
Ending SubTask 8
Ending SubTask 9
Ending SubTask 10
Program finished

See also

I have also writtent a blog post on Tasks that create more work which may give further insight into this area.