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POWhy is using a Func<> so much faster than using the new() constraint on a generic sequence creator
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  1. POWhy is using a Func<> so much faster than using the new() constraint on a generic sequence creator
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    copied!<p>Consider the following code...</p> <p>In my tests for a RELEASE (not debug!) x86 build on a Windows 7 x64 PC (Intel i7 3GHz) I obtained the following results:</p> <pre><code>CreateSequence() with new() took 00:00:00.9158071 CreateSequence() with creator() took 00:00:00.1383482 CreateSequence() with new() took 00:00:00.9198317 CreateSequence() with creator() took 00:00:00.1372920 CreateSequence() with new() took 00:00:00.9340462 CreateSequence() with creator() took 00:00:00.1447375 CreateSequence() with new() took 00:00:00.9344077 CreateSequence() with creator() took 00:00:00.1365162 </code></pre> <p>It seems that using a Func&lt;> to define a delegate to create new objects is more than 6 times faster than calling "new T()" directly. </p> <p>I find this slightly unexpected... I guess it's because of some inlining done by the Jitter, but I'd have thought that it would have been able to optimize the "new T()" just as well.</p> <p>Does anyone have an explanation for this?</p> <p>Maybe I'm making some mistake. (I've considered the effect the garbage collector might have, but rearranging the code and adding GC.Collect() and so on doesn't change the results significantly).</p> <p>Anyway, here's the code:</p> <pre><code>using System; using System.Collections.Generic; using System.Diagnostics; using System.Linq; namespace ConsoleApplication6 { class Program { static void Main(string[] args) { Stopwatch sw = new Stopwatch(); int repeats = 100; int count = 100000; for (int outer = 0; outer &lt; 4; ++outer) { sw.Restart(); for (int inner = 0; inner &lt; repeats; ++inner) { CreateSequence&lt;object&gt;(count).Count(); } Console.WriteLine("CreateSequence() with new() took " + sw.Elapsed); sw.Restart(); for (int inner = 0; inner &lt; repeats; ++inner) { CreateSequence(count, () =&gt; new object()).Count(); } Console.WriteLine("CreateSequence() with creator() took " + sw.Elapsed); Console.WriteLine(); } } public static IEnumerable&lt;T&gt; CreateSequence&lt;T&gt;(int n) where T: new() { for (int i = 0; i &lt; n; ++i) { yield return new T(); } } public static IEnumerable&lt;T&gt; CreateSequence&lt;T&gt;(int n, Func&lt;T&gt; creator) { for (int i = 0; i &lt; n; ++i) { yield return creator(); } } } } </code></pre>
 

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