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POWhy does IQueryable.All() return true on an empty collection?
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  1. POWhy does IQueryable.All() return true on an empty collection?
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    copied!<p>So I ran into a situation today where some production code was failing precisely because a method performed exactly as <a href="http://msdn.microsoft.com/en-us/library/bb534754.aspx" rel="noreferrer">documented in MSDN</a>. Shame on me for not reading the documentation. However, I'm still scratching my head as to <em>why</em> it behaves this way, even if "by design", since this behavior is exactly opposite what I would have expected (and other, known behaviors) and therefore seems to violate the principle of least surprise.</p> <p>The <code>All()</code> method allows you to supply a predicate (such as a lambda expression) to test an <code>IQueryable</code>, returning a Boolean value that indicates whether all collection members match the test. So far so good. Here's where it gets weird. <code>All()</code> also returns <code>true</code> if the collection is empty. This seems completely backwards to me, for the following reasons:</p> <ul> <li>If the collection is empty, a test like this is, at best, undefined. If my driveway is empty, I cannot assert that all cars parked there are red. With this behavior, on an empty driveway all cars parked there are red AND blue AND checkerboard - all of these expressions would return true.</li> <li>For anyone familiar with the SQL notion that NULL != NULL, this is unexpected behavior.</li> <li>The <code>Any()</code> method behaves as expected, and (correctly) returns false because it does not have any members that match the predicate.</li> </ul> <p>So my question is, why does <code>All()</code> behave this way? What problem does it solve? Does this violate the principle of least surprise?</p> <p>I tagged this question as .NET 3.5, though the behavior also applies to .NET 4.0 as well.</p> <p><strong>EDIT</strong> Ok, so I grasp the logic aspect to this, as so excellently laid out by Jason and the rest of you. Admittedly, an empty collection is something of an edge case. I guess my question is rooted in the struggle that, just because something is <em>logical</em> doesn't mean it necessarily makes <em>sense</em> if you're not in the correct frame of mind.</p>
 

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