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  1. // Program.cs
  2. using System;
  3. using System.Collections.Generic;
  4. using System.Diagnostics;
  5. using System.Linq;
  6.  
  7. namespace WhereSelectDemo
  8. {
  9.     class Branch
  10.     {
  11.         public Guid CompanyId { get; set; }
  12.         public int BranchCode { get; set; }
  13.         // (you could add more props if needed)
  14.     }
  15.  
  16.     class Program
  17.     {
  18.         static void Main()
  19.         {
  20.             try
  21.             {
  22.                 const int NumItems = 200_000; // adjust if your IDE/machine can't handle this (try 200_000)
  23.                 Console.WriteLine($"Creating {NumItems:N0} Branch items...");
  24.  
  25.                 var targetCompanyId = Guid.NewGuid();
  26.                 var otherCompanyId = Guid.NewGuid();
  27.  
  28.                 // Create list with ~25% matches to targetCompanyId
  29.                 var branches = new List<Branch>(NumItems);
  30.                 for (int i = 0; i < NumItems; i++)
  31.                 {
  32.                     branches.Add(new Branch
  33.                     {
  34.                         CompanyId = (i % 4 == 0) ? targetCompanyId : otherCompanyId, // ~25% matches
  35.                         BranchCode = i % 1000
  36.                     });
  37.                 }
  38.  
  39.                 Console.WriteLine($"Target CompanyId: {targetCompanyId}");
  40.                 Console.WriteLine("Running Where -> Select ...");
  41.  
  42.                 var sw = Stopwatch.StartNew();
  43.                 long hitCount1 = 0;
  44.  
  45.                 // Filter first, then project - selector runs only for filtered items
  46.                 var result1 = branches
  47.                     .Where(b =>
  48.                     {
  49.                         hitCount1++;               // counting the predicate calls
  50.                         return b.CompanyId == targetCompanyId;
  51.                     })
  52.                     .Select(b =>
  53.                     {
  54.                         hitCount1++;               // counting the selector calls (only for matching items)
  55.                         return new { b.CompanyId, b.BranchCode };
  56.                     })
  57.                     .ToArray(); // force evaluation
  58.                 sw.Stop();
  59.  
  60.                 Console.WriteLine($"Where -> Select: {sw.ElapsedMilliseconds} ms, hitCount = {hitCount1:N0}, result count = {result1.Length:N0}");
  61.  
  62.                 // ---- second experiment ----
  63.                 Console.WriteLine("Running Select -> Where ...");
  64.                 sw.Restart();
  65.                 long hitCount2 = 0;
  66.  
  67.                 // Project first, then filter - projector runs for all items, then predicate runs for all projected items
  68.                 var result2 = branches
  69.                     .Select(b =>
  70.                     {
  71.                         hitCount2++; // projector called for every element
  72.                         return new { b.CompanyId, b.BranchCode };
  73.                     })
  74.                     .Where(x =>
  75.                     {
  76.                         hitCount2++; // predicate called for every element of the projection
  77.                         return x.CompanyId == targetCompanyId;
  78.                     })
  79.                     .ToArray(); // force evaluation
  80.                 sw.Stop();
  81.  
  82.                 Console.WriteLine($"Select -> Where: {sw.ElapsedMilliseconds} ms, hitCount = {hitCount2:N0}, result count = {result2.Length:N0}");
  83.             }
  84.             catch (Exception ex)
  85.             {
  86.                 Console.WriteLine("Error: " + ex);
  87.             }
  88.  
  89.             Console.WriteLine("Done. Press any key to exit.");
  90.             Console.ReadKey();
  91.         }
  92.     }
  93. }
  94.  
Success #stdin #stdout 0.57s 184180KB
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stdin
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stdout
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Creating 2,000,000 Branch items...
Target CompanyId: 9831b8fa-0f64-4e84-86ce-fa01ef5e0a2e
Running Where -> Select ...
Where -> Select: 128 ms, hitCount = 2,500,000, result count = 500,000
Running Select -> Where ...
Select -> Where: 178 ms, hitCount = 4,000,000, result count = 500,000
Done. Press any key to exit.
https://ideone.com/ODiaRz
language:
C# (gmcs 5.20.1)
created:
18 hours ago
visibility:
public

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