using System;

using System.Collections.Generic;

using System.Diagnostics;

using System.Linq;

​

public class Program

{

      public static void Main(string[] args)

    {

        var userProfiles = GenerateUserProfiles();

        var idList = GenerateIds();

        var idsHashSet = idList.ToHashSet();

          var a = idsHashSet.Count;

        var stopWatch = new Stopwatch();

        stopWatch.Start();

        userProfiles.Join(idList, up => up.ID, id => id, (up, id) => up).ToArray();

        Console.WriteLine("Elapsed .Join with list time: {0}", stopWatch.Elapsed);

        stopWatch.Restart();

        userProfiles.Where(up => idList.Contains(up.ID)).ToArray();

        Console.WriteLine("Elapsed .Where .Contains with list time: {0}", stopWatch.Elapsed);

        Console.WriteLine("-----------------------------");

        stopWatch.Restart();

        userProfiles.Join(idsHashSet, up => up.ID, id => id, (up, id) => up).ToArray();

        Console.WriteLine("Elapsed .Join time with HashSet: {0}", stopWatch.Elapsed);

        stopWatch.Restart();

        userProfiles.Where(up => idsHashSet.Contains(up.ID)).ToArray();

        Console.WriteLine("Elapsed .Where2 .Contains with HashSet time: {0}", stopWatch.Elapsed);

        Console.ReadLine();

    }

​

    private static IEnumerable<int> GenerateIds()

    {

       // var result = new List<int>();

        for (int i = 10000; i > 0; i--)

        {

            yield return i;

        }

    }

​

    private static IEnumerable<Invoice> GenerateUserProfiles()

    {

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

        {

            yield return new Invoice {ID = i};

        }

    }

    public class Invoice

    {

        public int ID {get; set; }

    }

}