using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;

public class MyClass
{
    public int Id { get; private set; }
    public string Name { get; private set; }

    public MyClass(int id)
    {
        this.Id = id;
        this.Name = id.ToString();
    }
}

class Program
{
    static void Main(string[] args)
    {
        var rand = new Random();
        var arrayA = Enumerable.Range(1, 10000).Select(_ => new MyClass(rand.Next(1, 100))).ToArray();
        var arrayB = Enumerable.Range(1, 10000).Select(_ => new MyClass(rand.Next(1, 100))).ToArray();

        GC.Collect();
        GC.WaitForPendingFinalizers();
        GC.Collect();
        var sw = Stopwatch.StartNew();

        var ret = (
            from a in arrayA
            join b in arrayB on a.Id equals b.Id
            select a.Name).ToList();

        sw.Stop();
        Console.WriteLine(sw.Elapsed);

        GC.Collect();
        GC.WaitForPendingFinalizers();
        GC.Collect();
        sw.Reset(); sw.Start();

        var ret2 = new List<string>();
        var lookup = arrayB.ToLookup(x => x.Id);
        foreach (var itemA in arrayA)
        {
            foreach (var itemB in lookup[itemA.Id])
            {
                ret2.Add(itemA.Name);
            }
        }

        sw.Stop();
        Console.WriteLine(sw.Elapsed);

        Console.Write("check result : ");
        Console.WriteLine(ret.SequenceEqual(ret2));
    }
}