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POWhy does casting List<T> into IList<T> result in reduced performance?
 

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  1. POWhy does casting List<T> into IList<T> result in reduced performance?
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    <p>I was doing some performance metrics and I ran into something that seems quite odd to me. I time the following two functions:</p> <pre><code> private static void DoOne() { List&lt;int&gt; A = new List&lt;int&gt;(); for (int i = 0; i &lt; 200; i++) A.Add(i); int s=0; for (int j = 0; j &lt; 100000; j++) { for (int c = 0; c &lt; A.Count; c++) s += A[c]; } } private static void DoTwo() { List&lt;int&gt; A = new List&lt;int&gt;(); for (int i = 0; i &lt; 200; i++) A.Add(i); IList&lt;int&gt; L = A; int s = 0; for (int j = 0; j &lt; 100000; j++) { for (int c = 0; c &lt; L.Count; c++) s += L[c]; } } </code></pre> <p>Even when compiling in release mode, the timings results were consistently showing that DoTwo takes ~100 longer then DoOne:</p> <pre><code> DoOne took 0.06171706 seconds. DoTwo took 8.841709 seconds. </code></pre> <p>Given the fact the List directly implements IList I was very surprised by the results. Can anyone clarify this behavior?</p> <h2>The gory details</h2> <p>Responding to questions, here is the full code and an image of the project build preferences:</p> <blockquote class="spoiler"> <p> <a href="http://img178.imageshack.us/img178/9456/projpref.tif" rel="nofollow noreferrer">Dead Image Link</a></p> </blockquote> <pre><code>using System; using System.Collections.Generic; using System.Text; using System.Diagnostics; using System.Collections; namespace TimingTests { class Program { static void Main(string[] args) { Stopwatch SW = new Stopwatch(); SW.Start(); DoOne(); SW.Stop(); Console.WriteLine(" DoOne took {0} seconds.", ((float)SW.ElapsedTicks) / Stopwatch.Frequency); SW.Reset(); SW.Start(); DoTwo(); SW.Stop(); Console.WriteLine(" DoTwo took {0} seconds.", ((float)SW.ElapsedTicks) / Stopwatch.Frequency); } private static void DoOne() { List&lt;int&gt; A = new List&lt;int&gt;(); for (int i = 0; i &lt; 200; i++) A.Add(i); int s=0; for (int j = 0; j &lt; 100000; j++) { for (int c = 0; c &lt; A.Count; c++) s += A[c]; } } private static void DoTwo() { List&lt;int&gt; A = new List&lt;int&gt;(); for (int i = 0; i &lt; 200; i++) A.Add(i); IList&lt;int&gt; L = A; int s = 0; for (int j = 0; j &lt; 100000; j++) { for (int c = 0; c &lt; L.Count; c++) s += L[c]; } } } } </code></pre> <p>Thanks for all the good answers (especially @kentaromiura). I would have closed the question, though I feel we still miss an important part of the puzzle. Why would accessing a class via an interface it implements be so much slower? The only difference I can see is that accessing a function via an Interface implies using virtual tables while the normally the functions can be called directly. To see whether this is the case I made a couple of changes to the above code. First I introduced two almost identical classes:</p> <pre><code> public class VC { virtual public int f() { return 2; } virtual public int Count { get { return 200; } } } public class C { public int f() { return 2; } public int Count { get { return 200; } } } </code></pre> <p>As you can see VC is using virtual functions and C doesn't. Now to DoOne and DoTwo:</p> <pre><code> private static void DoOne() { C a = new C(); int s=0; for (int j = 0; j &lt; 100000; j++) { for (int c = 0; c &lt; a.Count; c++) s += a.f(); } } private static void DoTwo() { VC a = new VC(); int s = 0; for (int j = 0; j &lt; 100000; j++) { for (int c = 0; c &lt; a.Count; c++) s += a.f(); } } </code></pre> <p>And indeed:</p> <pre><code>DoOne took 0.01287789 seconds. DoTwo took 8.982396 seconds. </code></pre> <p>This is even more scary - virtual function calls 800 times slower?? so a couple of question to the community:</p> <ol> <li>Can you reproduce? (given the fact that all had worse performance before, but not as bad as mine)</li> <li>Can you explain? </li> <li>(this may be the most important) - can you think of a way to avoid?</li> </ol> <p>Boaz</p>
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    1. COdoOne and doTwo are not equivalent, why you do this: List<int> A = new List<int>(); for (int i = 0; i < 200; i++) A.Add(i); IList<int> L = A; and not directly this: IList<int> A = new List<int(); for (int i = 0; i < 200; i++) A.Add(i); ????
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    2. CO@kentaromiura, When I ran my tests I used the version that you're suggesting. In my case it made very little difference to the results whether I used Boaz's original version or the corrected version. Your own results back this up too.
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    3. COYes, true, see my Answer ;)
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