using System;

using System.Threading.Tasks;

using System.Threading;

​

public class Program

{

​

    public static void Main()

    {       

        AutoResetEvent waitA = new AutoResetEvent(false);

        AutoResetEvent waitB = new AutoResetEvent(false);

        // Start tasks

        Task task_1 = Task.Run(() =>

                                       {

                                           Console.WriteLine("1");

                                           waitA.Set();

                                           waitB.WaitOne();

                                           Console.WriteLine("3");

                                       });

        Task task_2 = Task.Run(() =>

                                       {

                                           waitA.WaitOne();

                                           Console.WriteLine("2");

                                           waitB.Set();

                                       });

        task_2.Wait();

    }

}