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POC# Count() Extension Method Performance
 

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    <p>If the LINQ <code>Count()</code> extension method is invoked on an <code>IEnumerable&lt;T&gt;</code> that has a <code>Count</code> property (e.g. <code>List&lt;T&gt;</code>), does the <code>Count()</code> method look for that property and return it (rather than counting the items by enumerating them)? The following test code seems to indicate that it does:</p> <pre><code>using System; using System.Collections; using System.Collections.Generic; using System.Diagnostics; using System.Linq; namespace CountSpeedTest { // Output: // List - CLR : 0 ms // Enumerate - CLR : 10 ms // List - Mine: 12 ms // Enumerate - Mine: 12 ms class Program { private const int Runs = 10; private const int Items = 1000000; static void Main(string[] args) { var total = new long[] {0, 0, 0, 0}; for (int i = 0; i &lt; Runs; ++i) { var items = Enumerable.Range(0, Items).Select(o =&gt; o.ToString()).ToList(); var list = new List&lt;string&gt;(items); var enumerate = new Enumerate&lt;string&gt;(items); total[0] += TimeCount(list, c =&gt; c.Count()); total[1] += TimeCount(enumerate, c =&gt; c.Count()); total[2] += TimeCount(list, c =&gt; c.SlowCount()); total[3] += TimeCount(enumerate, c =&gt; c.SlowCount()); } Console.WriteLine(String.Format("List - CLR : {0} ms", total[0] / Runs)); Console.WriteLine(String.Format("Enumerate - CLR : {0} ms", total[1] / Runs)); Console.WriteLine(String.Format("List - Mine: {0} ms", total[2] / Runs)); Console.WriteLine(String.Format("Enumerate - Mine: {0} ms", total[3] / Runs)); Console.ReadKey(true); } private static long TimeCount&lt;T&gt;(IEnumerable&lt;T&gt; collection, Func&lt;IEnumerable&lt;T&gt;, int&gt; counter) { var stopwatch = Stopwatch.StartNew(); var count = counter(collection); stopwatch.Stop(); if (count != Items) throw new Exception("Incorrect Count"); return stopwatch.ElapsedMilliseconds; } } public static class CountExtensions { // Performs a simple enumeration based count. public static int SlowCount&lt;T&gt;(this IEnumerable&lt;T&gt; items) { var i = 0; var enumerator = items.GetEnumerator(); while (enumerator.MoveNext()) i++; return i; } } // Wraps an IEnumerable&lt;T&gt; to hide its Count property. public class Enumerate&lt;T&gt; : IEnumerable&lt;T&gt; { private readonly IEnumerable&lt;T&gt; collection; public Enumerate(IEnumerable&lt;T&gt; collection) { this.collection = collection; } public IEnumerator&lt;T&gt; GetEnumerator() { return collection.GetEnumerator(); } IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); } } } </code></pre> <p><strong>On a related note:</strong> how can a custom collection which implements <code>IEnumerable&lt;T&gt;</code> expose its own <code>Count</code> property in such a way that the CLR <code>Count()</code> extension method can take advantage of it?</p>
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