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    <p>I believe it is an issue of covariance or contravariance as noted by <a href="http://www.manning-sandbox.com/message.jspa?messageID=77137#77137" rel="noreferrer">this forum post</a>.</p> <p>See <a href="http://blogs.msdn.com/ericlippert/archive/2007/10/17/covariance-and-contravariance-in-c-part-two-array-covariance.aspx" rel="noreferrer">Covariance and Contravariance in C#, Part Two: Array Covariance</a> and the rest of the <a href="http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx" rel="noreferrer">Covariance and Contravariance series</a> at Eric Lippert's blog.</p> <p>Although he is dealing with Arrays in the article I linked, I believe a similar problem presents itself here. With your first example, you are returning an <code>IEnumerable</code> that could contain objects that implement an interface that is <em>larger</em> than <code>ISomeTable</code> (i.e. - you could put a Turtle into an Animals IEnumerable when that IEnumerable can only contain Giraffes). I think the reason it works when you return <code>IQueryable</code> is because that is <em>larger/wider</em> than anything you could return, so you're guaranteed that what you return you will be able to handle(?).</p> <p>In the second example, <a href="http://msdn.microsoft.com/en-us/library/bb360913.aspx" rel="noreferrer">OfType</a> is ensuring that what gets returned is an object that stores all the information necessary to return only those elements that can be cast to Giraffe.</p> <p>I'm pretty sure it has something to do with the issues of type safety outlined above, but as Eric Lippert says <a href="http://blogs.msdn.com/ericlippert/archive/2007/10/24/covariance-and-contravariance-in-c-part-five-higher-order-functions-hurt-my-brain.aspx" rel="noreferrer">Higher Order Functions Hurt My Brain</a> and I am having trouble expressing precisely why this is a co/contravariant issue.</p>
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    1. USGrant Wagner
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    1. COThis sounds right to me. I can see where you're coming from on not being able to express why this is. I'll wait a while and see if anyone has a more direct explanation, otherwise I'll accept this answer.
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      1. USJason Baker
 

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