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POOrder of LINQ extension methods does not affect performance?
 

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  1. POOrder of LINQ extension methods does not affect performance?
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    <p>I'm surprised that it apparently doesn't matter whether i prepend or append LINQ extension methods. </p> <p>Tested with <a href="http://msdn.microsoft.com/en-us/library/bb340482.aspx"><code>Enumerable.FirstOrDefault</code></a>:</p> <ol> <li><code>hugeList.Where(x =&gt; x.Text.Contains("10000")).FirstOrDefault();</code></li> <li><p><code>hugeList.FirstOrDefault(x =&gt; x.Text.Contains("10000"));</code></p> <pre><code>var hugeList = Enumerable.Range(1, 50000000) .Select(i =&gt; new { ID = i, Text = "Item" + i }); var sw1 = new System.Diagnostics.Stopwatch(); var sw2 = new System.Diagnostics.Stopwatch(); sw1.Start(); for(int i=0;i&lt;1000;i++) hugeList.Where(x =&gt; x.Text.Contains("10000")).FirstOrDefault(); sw1.Stop(); sw2.Start(); for(int i=0;i&lt;1000;i++) hugeList.FirstOrDefault(x =&gt; x.Text.Contains("10000")); sw2.Stop(); var result1 = String.Format("FirstOrDefault after: {0} FirstOrDefault before: {1}", sw1.Elapsed, sw2.Elapsed); //result1: FirstOrDefault after: 00:00:03.3169683 FirstOrDefault before: 00:00:03.0463219 sw2.Restart(); for (int i = 0; i &lt; 1000; i++) hugeList.FirstOrDefault(x =&gt; x.Text.Contains("10000")); sw2.Stop(); sw1.Restart(); for (int i = 0; i &lt; 1000; i++) hugeList.Where(x =&gt; x.Text.Contains("10000")).FirstOrDefault(); sw1.Stop(); var result2 = String.Format("FirstOrDefault before: {0} FirstOrDefault after: {1}", sw2.Elapsed, sw1.Elapsed); //result2: FirstOrDefault before: 00:00:03.6833079 FirstOrDefault after: 00:00:03.1675611 //average after:3.2422647 before: 3.3648149 (all seconds) </code></pre></li> </ol> <p>I would have guessed that it would be slower to prepend <code>Where</code> since it must find all matching items and then take the first and a preceded <code>FirstOrDefault</code> could yield the first found item. </p> <p><strong>Q:</strong> Can somebody explain why i'm on the wrong track? </p>
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    1. USRobert Harvey
    1. USTim Schmelter
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    1. COpossible duplicate of [What are the benefits of a Deferred Execution in LINQ?](http://stackoverflow.com/questions/7324033/what-are-the-benefits-of-a-deferred-execution-in-linq)
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      1. USKirk Woll
    2. CO@KirkWoll: How is that even remotely a duplicate? The OP here is asking about order of operations; your cited duplicate is asking about deferred execution.
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      1. USRobert Harvey
    3. CO@Robert, I don't think it's anything fancier than that `.FirstOrDefault` evaluates the deferred query constrained by the `.Where` and consumes only one result. Becasue of deferred execution, the entire contents of the sequence does not have to be evaluated. To me, it's a duplicate because I assumed the OP here didn't know about deferred execution. This behavior seems obvious to me once you have that knowledge.
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