A simple ThreadPool implementation. Not to be used, but to explain the concepts.

ThreadPool.cs

class ThreadPool
{
  private static readonly object LockObject = new object();
	private static readonly Queue<Action> _work = new Queue<Action>();
	private static readonly List<Thread> _workers = new List<Thread>();

	static ThreadPool()
	{
		for (var i = 0; i < 5; i++)
		{
			var thread = new Thread(DoWork);

			_workers.Add(thread);
		}
	}

	public void QueueWork(Action action)
	{
		lock (LockObject)
		{
			_work.Enqueue(action);
		}
	}

	private static void DoWork()
	{
		while (true)
		{
			Action action;

			lock (LockObject)
			{
				action = _work.Dequeue();
			}

			if (action != null)
			{
				action();
			}

			Thread.Sleep(1);
		}
	}
}