AutoResetEvent

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;

namespace WaitOne
{
    class Program
    {
        static AutoResetEvent autoEvent = new AutoResetEvent(false);
        static void Main(string[] args)
        {
            Console.WriteLine("Main starting.");

            ThreadPool.QueueUserWorkItem(
                new WaitCallback(WorkMethod), autoEvent);

            // Wait for work method to signal.
            autoEvent.WaitOne();
            Console.WriteLine("Work method signaled.
Main ending.");

        }

        static void WorkMethod(object stateInfo)
        {
            Console.WriteLine("Work starting.");

            // Simulate time spent working.
            Thread.Sleep(new Random().Next(100, 2000));

            // Signal that work is finished.
            Console.WriteLine("Work ending.");
            ((AutoResetEvent)stateInfo).Set();
        }

    }
}