using System;

using System.Threading;

using System.Threading.Tasks;

public class Program

{

    public static void Main()

    {

        Old();

        Console.WriteLine("=====");

        Better();

    }

    public static void Old()

    {

        var a = Task.Run(()=>

        {

            Thread.Sleep(50);

            var started = DateTime.UtcNow;

            Console.WriteLine("a run {0}",started.Ticks);

        });

        var b = Task.Run(()=>

        {

            Thread.Sleep(40);

            var started = DateTime.UtcNow;

            Console.WriteLine("b run {0}",started.Ticks);

        });

        Task.WaitAll(new Task[]{a,b});

    }

    public static void Better()

    {

        Console.WriteLine("prepare");

        var readyToRace = new CountdownEvent(2);

        var fireTheGun = new ManualResetEvent(false);

        var a = Task.Run(()=>

        {

            Thread.Sleep(50);

            readyToRace.Signal();

            fireTheGun.WaitOne();

            var started = DateTime.UtcNow;

            Console.WriteLine("a run {0}",started.Ticks);

        });

        var b = Task.Run(()=>

        {

            Thread.Sleep(40);

            readyToRace.Signal();

            fireTheGun.WaitOne();

            var started = DateTime.UtcNow;

            Console.WriteLine("b run {0}",started.Ticks);

        });

        readyToRace.Wait();

        Console.WriteLine("ready");

        fireTheGun.Set();

        Task.WaitAll(new Task[]{a,b});

    }

}