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    <p>Funny thing. Do you know how is <code>Sum(this IEnumerable&lt;TSource&gt; source, Func&lt;TSource, int&gt; selector)</code> defined? <strong>It uses <code>Select</code> method!</strong></p> <pre><code>public static int Sum&lt;TSource&gt;(this IEnumerable&lt;TSource&gt; source, Func&lt;TSource, int&gt; selector) { return source.Select(selector).Sum(); } </code></pre> <p>So actually, it all should work nearly the same. I did quick research on my own, and here are the results:</p> <pre><code>Where -- mod: 1 result: 0, time: 371 ms WhereSelect -- mod: 1 result: 0, time: 356 ms Select -- mod: 1 result 0, time: 366 ms Sum -- mod: 1 result: 0, time: 363 ms ------------- Where -- mod: 2 result: 4999999, time: 469 ms WhereSelect -- mod: 2 result: 4999999, time: 429 ms Select -- mod: 2 result 4999999, time: 362 ms Sum -- mod: 2 result: 4999999, time: 358 ms ------------- Where -- mod: 3 result: 9999999, time: 441 ms WhereSelect -- mod: 3 result: 9999999, time: 452 ms Select -- mod: 3 result 9999999, time: 371 ms Sum -- mod: 3 result: 9999999, time: 380 ms ------------- Where -- mod: 4 result: 7500000, time: 571 ms WhereSelect -- mod: 4 result: 7500000, time: 501 ms Select -- mod: 4 result 7500000, time: 406 ms Sum -- mod: 4 result: 7500000, time: 397 ms ------------- Where -- mod: 5 result: 7999999, time: 490 ms WhereSelect -- mod: 5 result: 7999999, time: 477 ms Select -- mod: 5 result 7999999, time: 397 ms Sum -- mod: 5 result: 7999999, time: 394 ms ------------- Where -- mod: 6 result: 9999999, time: 488 ms WhereSelect -- mod: 6 result: 9999999, time: 480 ms Select -- mod: 6 result 9999999, time: 391 ms Sum -- mod: 6 result: 9999999, time: 387 ms ------------- Where -- mod: 7 result: 8571428, time: 489 ms WhereSelect -- mod: 7 result: 8571428, time: 486 ms Select -- mod: 7 result 8571428, time: 384 ms Sum -- mod: 7 result: 8571428, time: 381 ms ------------- Where -- mod: 8 result: 8749999, time: 494 ms WhereSelect -- mod: 8 result: 8749999, time: 488 ms Select -- mod: 8 result 8749999, time: 386 ms Sum -- mod: 8 result: 8749999, time: 373 ms ------------- Where -- mod: 9 result: 9999999, time: 497 ms WhereSelect -- mod: 9 result: 9999999, time: 494 ms Select -- mod: 9 result 9999999, time: 386 ms Sum -- mod: 9 result: 9999999, time: 371 ms </code></pre> <p>For following implementations:</p> <pre><code>result = source.Where(x =&gt; x.IsValid).Sum(x =&gt; x.Value); result = source.Select(x =&gt; x.IsValid ? x.Value : 0).Sum(); result = source.Sum(x =&gt; x.IsValid ? x.Value : 0); result = source.Where(x =&gt; x.IsValid).Select(x =&gt; x.Value).Sum(); </code></pre> <p><code>mod</code> means: every 1 from <code>mod</code> items is invalid: for <code>mod == 1</code> every item is invalid, for <code>mod == 2</code> odd items are invalid, etc. Collection contains <code>10000000</code> items.</p> <p><img src="https://i.stack.imgur.com/ET6C6.png" alt="enter image description here"></p> <p>And results for collection with <code>100000000</code> items:</p> <p><img src="https://i.stack.imgur.com/mdF6L.png" alt="enter image description here"></p> <p>As you can see, <code>Select</code> and <code>Sum</code> results are quite consistent across all <code>mod</code> values. However <code>where</code> and <code>where</code>+<code>select</code> are not.</p>
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    1. COIt's very interesting that in your results, all methods starts at the same place and diverge, whereas the It'sNotALie's results cross in the middle.
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