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POIf I cast an IQueryable as an IEnumerable then call a Linq extension method, which implementation gets called?
 

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  1. POIf I cast an IQueryable as an IEnumerable then call a Linq extension method, which implementation gets called?
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    <p>Considering the following code:</p> <pre><code>IQueryable&lt;T&gt; queryable; // something to instantiate queryable var enumerable = (IEnumerable&lt;T&gt;) queryable; var filtered = enumerable.Where(i =&gt; i &gt; 3); </code></pre> <p>In the final line, which extension method gets called? </p> <p>Is it <code>IEnumerable&lt;T&gt;.Where(...)</code>? Or will <code>IQueryable&lt;T&gt;.Where(...)</code> be called because the actual implementation is still obviously a queryable?</p> <p>Presumably the ideal would be for the IQueryable version to be called, in the same way that normal polymorphism will always use the more specific override.</p> <p>In Visual Studio though when I right-click on the Where method and "Go to Definition" I'm taken to the IEnumerable version, which kind of makes sense from a visual point-of-view.</p> <p>My main concern is that if somewhere in my app I use Linq to NHibernate to get a Queryable, but I pass it around using an interface that uses the more general IEnumerable signature, I'll lose the wonders of deferred database execution!</p> <hr> <p>Edit: It turns out that, as Iridium has pointed out, it's the Enumerable version that gets called</p> <pre><code>public class Program { static void Main(string[] args) { var myString2 = new MyString2(); var myString = (MyString)myString2; Console.WriteLine(myString.Method()); Console.ReadLine(); } } public class MyString {} public class MyString2 : MyString {} public static class ExtensionMethods { public static string Method(this MyString instance) { return "MyString method"; } public static string Method(this MyString2 instance) { return "MyString2 method"; } } </code></pre> <p>The output is "MyString method".</p>
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    1. USJames Morcom
    1. USJames Morcom
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      1. POIf I cast an IQueryable as an IEnumerable then call a Linq extension method, which implementation gets called?
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    1. COForgot to vote up the question... it was interesting to find out :).
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      1. POIf I cast an IQueryable as an IEnumerable then call a Linq extension method, which implementation gets called?
      1. USTomas Jansson
    2. COhttp://stackoverflow.com/questions/474074/overriding-extension-methods
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      1. POIf I cast an IQueryable as an IEnumerable then call a Linq extension method, which implementation gets called?
      1. USJahan Zinedine
    3. CORemember, extension method syntax is just a syntactic sugar for *calling a static method* and static methods by definition *use static analysis to determine what method to call*. If you want *runtime* analysis, you'll need to write the code yourself to do that. Moreover: it is *not* considered desirable for an IQ that is statically an IE to unexpectedly morph back to the IQ code path; presumably you cast it to an IE because you *don't* want to treat it as an IQ! If you want to treat it as an IQ then *don't cast it away*. The Groucho rule: if it hurts when you do that, *don't do that*.
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      1. USEric Lippert
 

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