gistfile1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections.Concurrent;
using System.Threading;

namespace ConsoleApplication1
{
    class Program
    {
        static ConcurrentDictionary<int, string> dict = new ConcurrentDictionary<int, string>(); 

        static void Main(string[] args)
        {
            var rand = new Random(); 
            new Thread(new ThreadStart(() => 
            {
                while (true)
                {
                    dict.AddOrUpdate(rand.Next(10000), "a", (k, current) => "b");
                }
            })).Start();

            new Thread(new ThreadStart(() =>
            {
                while (true)
                {
                    string ignore;
                    dict.TryRemove(rand.Next(1000), out ignore);
                }
            })).Start();

            new Thread(new ThreadStart(() =>
            {
                while (true)
                {
                    Console.WriteLine("size is " + dict.Count);
                    int a = 0;
                    int b = 0; 
                    foreach (var pair in dict)
                    {
                        a += pair.Value == "a" ? 1 : 0;
                        b += pair.Value == "b" ? 1 : 0;
                    }
                    Console.WriteLine("a: " + a + "  b: " + b);
                    Thread.Sleep(100);
                }
            })).Start();

            Console.ReadKey();
        }

    }
}