using System;

using System.Collections.Generic;           

using System.Threading.Tasks;

using System.Collections.Concurrent;

public class Program

{

​

    public static void Main()

    {

        var items = new List<int>();

​

        var tasks = new List<Task>();

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

        {

            tasks.Add(Task.Run(() => {

                for (int k = 0; k < 10000; k++)

                {

                    items.Add(i);

                }

            }));

        }

        Task.WaitAll(tasks.ToArray());

        Console.WriteLine(items.Count);

    }

}