Program.cs

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

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            Task.Run(MainAsync).Wait() ;

        }

        private static async Task MainAsync()
        {
            var stream = File.OpenWrite("test6.txt");

            List<Task> tasks = new List<Task>();
            Random rnd = new Random();
            foreach (var value in Enumerable.Range(1,100))
            {
                var delay = rnd.Next(1000);
                tasks.Add( WriteInt(value,stream,delay) );
            }
            Task.WaitAll(tasks.ToArray());

            stream.Close();
        }

        private static async Task WriteInt(int value, FileStream stream, int delay)
        {
            await Task.Delay(delay).ConfigureAwait(false);
            var text = $"WriteInt: {value} {Environment.NewLine}";
            var bytes = Encoding.ASCII.GetBytes(text);
            await stream.WriteAsync(bytes, 0, bytes.Length);
        }
    }
}