using System;

using System.Collections.Generic;

using System.Threading;     

using System.Threading.Tasks;       

​

public class Program

{

    public static void Main()

    {

        var semaphore = new Semaphore(1, 1);

        var tasks = new List<Task>();

        for (var i = 0; i < 10; i++)

        {

            var localI = i;

            var task = Task.Run(async () =>

                                {

                                    semaphore.WaitOne();

                                    try

                                    {

                                        await Task.Delay(100);

                                        Console.WriteLine("{0}: {1:o}", localI, DateTimeOffset.UtcNow);

                                    }

                                    finally

                                    {

                                        semaphore.Release();    

                                    }

                                });

            tasks.Add(task);

        }

        Task.WhenAll(tasks).GetAwaiter().GetResult();

    }

}