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  1. using System;
  2. using System.Collections.Generic;
  3. using System.Diagnostics;
  4. using System.Linq;
  5.  
  6. public class MyClass
  7. {
  8.     public int Id { get; private set; }
  9.     public string Name { get; private set; }
  10.  
  11.     public MyClass(int id)
  12.     {
  13.         this.Id = id;
  14.         this.Name = id.ToString();
  15.     }
  16. }
  17.  
  18. class Program
  19. {
  20.     static void Main(string[] args)
  21.     {
  22.         var rand = new Random();
  23.         var arrayA = Enumerable.Range(1, 10000).Select(_ => new MyClass(rand.Next(1, 100))).ToArray();
  24.         var arrayB = Enumerable.Range(1, 10000).Select(_ => new MyClass(rand.Next(1, 100))).ToArray();
  25.  
  26.         GC.Collect();
  27.         GC.WaitForPendingFinalizers();
  28.         GC.Collect();
  29.         var sw = Stopwatch.StartNew();
  30.  
  31.         var ret = (
  32.             from a in arrayA
  33.             join b in arrayB on a.Id equals b.Id
  34.             select a.Name).ToList();
  35.  
  36.         sw.Stop();
  37.         Console.WriteLine(sw.Elapsed);
  38.  
  39.         GC.Collect();
  40.         GC.WaitForPendingFinalizers();
  41.         GC.Collect();
  42.         sw.Reset(); sw.Start();
  43.  
  44.         var ret2 = new List<string>();
  45.         var lookup = arrayB.ToLookup(x => x.Id);
  46.         foreach (var itemA in arrayA)
  47.         {
  48.             foreach (var itemB in lookup[itemA.Id])
  49.             {
  50.                 ret2.Add(itemA.Name);
  51.             }
  52.         }
  53.  
  54.         sw.Stop();
  55.         Console.WriteLine(sw.Elapsed);
  56.  
  57.         Console.Write("check result : ");
  58.         Console.WriteLine(ret.SequenceEqual(ret2));
  59.     }
  60. }
stdin
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Standard input is empty
compilation info 
prog.cs(48,26): warning CS0168: The variable `itemB' is declared but never used
Compilation succeeded - 1 warning(s)
stdout
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00:00:00.1688902
00:00:00.0924882
check result : True
https://ideone.com/HkAaw
language:
C# (gmcs 5.20.1)
created:
13 years ago
visibility:
public

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