using System;

using System.Threading;

public class Program {

    async static void Because(SemaphoreSlim barrier) {

        var start = DateTime.Now;

        for (int i=0; i<5; ++i) {

            await barrier.WaitAsync();

            Console.WriteLine($"Processing step {i} at {DateTime.Now-start}");

        }

    }

    public static void Main() {

        Console.WriteLine("Hello World");

        var barrier = new SemaphoreSlim(0); // don't let anyone in yet

        Because(barrier);

        for (int i=0; i<5; ++i) {

            //Thread.Yield(); // :shrug:

            Thread.Sleep(250);

            barrier.Release(); // let one in

        }

    }

}

​