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    <p>There's no existing extension method like what you have. Let me explain why I think that is (aside from the obvious "because it wasn't specified, implemented, tested, documented, etc." reason).</p> <p>Basically, this implementation is necessarily inefficient. Constructing an array from the parameters passed to <code>In</code> (as happens when you use the <code>params</code> keyword) is an O(N) operation and causes gratuitous GC pressure (from the construction of a new <code>T[]</code> object). <code>Contains</code> then enumerates over that array, which means your original code has been more than doubled in execution time (instead of one partial enumeration via short-circuited evaluation, you've got one full enumeration followed by a partial enumeration).</p> <p>The GC pressure caused by the array construction <em>could</em> be alleviated somewhat by replacing the <code>params</code> version of the extension method with X overloads taking from 1 to X parameters of type <code>T</code> where X is some reasonable number... like 1-2 dozen. But this does not change the fact that you're passing X values onto a new level of the call stack only to check potentially less than X of them (i.e., it does not eliminate the performance penalty, only reduces it).</p> <p>And then there's another issue: if you intend for this <code>In</code> extension method to serve as a replacement for a bunch of chained <code>||</code> comparisons, there's something else you might be overlooking. With <code>||</code>, you get short-circuited evaluation; the same doesn't hold for parameters passed to methods. In the case of an enum, like in your example, this doesn't matter. But consider this code:</p> <pre><code>if (0 == array.Length || 0 == array[0].Length || 0 == array[0][0].Length) { // One of the arrays is empty. } </code></pre> <p>The above (weird/bad -- for illustration only) code should not throw an <code>IndexOutOfRangeException</code> (it could throw a <code>NullReferenceException</code>, but that's irrelevant to the point I'm making). However, the "equivalent" code using <code>In</code> very well could:</p> <pre><code>if (0.In(array.Length, array[0].Length, array[0][0].Length) { // This code will only be reached if array[0][0].Length == 0; // otherwise an exception will be thrown. } </code></pre> <p>I'm not saying your <code>In</code> extension idea is a bad one. In most cases, where used properly, it can save on typing and the performance/memory cost will not be noticeable. I'm just offering my thoughts on why a method of this sort would not be appropriate as a built-in library method: because its costs and limitations would likely be misunderstood, leading to over-use and suboptimal code.</p>
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    1. POC# In() method? (like Sql)
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    1. USDan Tao
    1. USDan Tao
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    1. COHow do you know is O(N)?
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      1. USmathk
    2. CO@mathk - Well, if you have n items, you have to add each of those n items to the array. There's no way you can ignore items; if you do, you haven't created an array of all n items by definition. So it has to be at least O(n).
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    3. COSince it is an immutable array you can leave it on he stack no need to move items. Donno how C# have make it out but it could be O(1)
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