c# - Parallel.For not to use my main thread

Question

In my application I want my main thread to be not used by anything else. I have to do some parallel processing that I would like to be done by different threads. For that I am using Parallel.For as follows

static void SomeMethod()
{
    Console.WriteLine(string.Format("Main Thread ID  before parallel loop ->>>>>>> {0} ", System.Threading.Thread.CurrentThread.ManagedThreadId));
    Parallel.For(0, 10, i =>
    {
        Console.WriteLine(string.Format("Output ->>>>>>> {0} ", System.Threading.Thread.CurrentThread.ManagedThreadId));
    }); 
    Thread.Sleep(100);
    Console.WriteLine(string.Format("Main Thread ID  after parallel loop ->>>>>>> {0} ", System.Threading.Thread.CurrentThread.ManagedThreadId));
}

As you can see from the output main thread is using ThreadID 1 and some threads from Parallel.For are also using same thread.

Main Thread ID  before parallel loop ->>>>>>> 1
Output ->>>>>>> 1
Output ->>>>>>> 1
Output ->>>>>>> 3
Output ->>>>>>> 4
Output ->>>>>>> 4
Output ->>>>>>> 4
Output ->>>>>>> 4
Output ->>>>>>> 5
Output ->>>>>>> 3
Output ->>>>>>> 1
Main Thread ID  after parallel loop ->>>>>>> 1

Is there some way to make sure that anything in Parallel.For always run on separate thread so that main thread is always free.

Answer 1

Is there some way to make sure that anything in Parallel.For always run on separate thread so that main thread is always free.

Parallel.For will always block until everything is finished - so even if it didn't do anything on the original thread, the thread still wouldn't be "free".

If you want to keep the main thread "free" you might want to look into async and await - you could use Task.Run to start the 10 tasks in an async method, and then await the result of calling Task.WhenAll.

Alternatively, you could still use Parallel.For but do that in a task. For example:

Task task = Task.Run(() => Parallel.For(0, 10, i =>
{
    Console.WriteLine("Output ->>>>>>> {0} ", 
                      Thread.CurrentThread.ManagedThreadId);
}));

You could then await that task. The "main thread" of the task will probably be used in the Parallel.For loop, but that's okay because it's still not your original main thread, if you see what I mean.