A bit of fun with Grand Central Dispatch...

GCDTest.cs

using System;
using System.Threading.Tasks;
using MonoMac.CoreFoundation;
using MonoMac.AppKit;
using System.Threading;

namespace GCDTest
{
	class MainClass
	{
		public static void Main (string[] args)
		{
			NSApplication.Init ();
		
			var gcds = new GrandCentralDispatchTaskScheduler ();
			var tf = new TaskFactory (gcds);
			tf.StartNew (() => {
				Console.WriteLine ("Hello, world.");
				Console.WriteLine ("Is thread pool thread: {0}", Thread.CurrentThread.IsThreadPoolThread);
				Console.WriteLine ("Dispatch queue: {0}", DispatchQueue.CurrentQueue.Label);
			}).Wait (Timeout.Infinite);

			Console.WriteLine ();

			Task.Factory.StartNew (() => {
				Console.WriteLine ("Hello, world from non-GCD task.");
				Console.WriteLine ("Is thread pool thread: {0}", Thread.CurrentThread.IsThreadPoolThread);
				Console.WriteLine ("Dispatch queue: {0}", DispatchQueue.CurrentQueue.Label);
			}).Wait (Timeout.Infinite);
		}
	}

	public class GrandCentralDispatchTaskScheduler : TaskScheduler
	{
		static readonly DispatchQueue gcdDispatchQueue = new DispatchQueue ("org.mono.tpl_dispatch_queue");

		protected override System.Collections.Generic.IEnumerable<Task> GetScheduledTasks ()
		{
			throw new NotImplementedException ();
		}

		protected override void QueueTask (Task task)
		{
			gcdDispatchQueue.DispatchAsync (delegate { 
				TryExecuteTaskInline (task, false);
			});
		}

		// TODO: Tasks cannot be dequeued.
		protected override bool TryDequeue (Task task)
		{
			return false;
		}

		protected override bool TryExecuteTaskInline (Task task, bool taskWasPreviouslyQueued)
		{
			if (taskWasPreviouslyQueued && !TryDequeue (task))
				return false;

			return TryExecuteTask (task);
		}
	}
}

output.txt

bojanrajkovic@dynamic-033 ~/C/O/G/G/b/Debug $ mono --debug GCDTest.exe
Hello, world.
Is thread pool thread: False
Dispatch queue: org.mono.tpl_dispatch_queue

Hello, world from non-GCD task.
Is thread pool thread: True
Dispatch queue: com.apple.root.default-overcommit-priority