using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Dictionary<string,bool> dict = new Dictionary<string,bool>(100000);

        for (int i = 0; i < 100000; i++)

        {

            dict.Add(System.Guid.NewGuid().ToString(), true);

        }

        var watch = new System.Diagnostics.Stopwatch();

        watch.Start();

        // find 100 values;

        for (int i = 0; i < 100; i++)

        {

            bool exists = dict.ContainsKey(System.Guid.NewGuid().ToString());

        }

​

        watch.Stop();

        Console.WriteLine("Execution Time: " + watch.ElapsedMilliseconds+ " ms");

    }

}