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POWhen is ObjectQuery really an IOrderedQueryable?
 

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  1. POWhen is ObjectQuery really an IOrderedQueryable?
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    <p>Applied to entity framework, the extension methods <code>Select()</code> and <code>OrderBy()</code> both return an <code>ObjectQuery</code>, which is defined as:</p> <pre><code>public class ObjectQuery&lt;T&gt; : ObjectQuery, IOrderedQueryable&lt;T&gt;, IQueryable&lt;T&gt;, &lt;... more interfaces&gt; </code></pre> <p>The return type of <code>Select()</code> is <code>IQueryable&lt;T&gt;</code> and that of <code>OrderBy</code> is <code>IOrderedQueryable&lt;T&gt;</code>. So you could say that both return the same type but in a different wrapper. Luckily so, because now we can apply <code>ThenBy</code> after <code>OrderBy</code> was called.</p> <p>Now my problem.</p> <p>Let's say I have this:</p> <pre><code>var query = context.Plots.Where(p =&gt; p.TrialId == 21); </code></pre> <p>This gives me an <code>IQueryable&lt;Plot&gt;</code>, which is an <code>ObjectQuery&lt;Plot&gt;</code>. But it is also an IOrderedQueryable:</p> <pre><code>var b = query is IOrderedQueryable&lt;Plot&gt;; // True! </code></pre> <p>But still:</p> <pre><code>var query2 = query.ThenBy(p =&gt; p.Number); // Does not compile. // 'IQueryable&lt;Plot&gt;' does not contain a definition for 'ThenBy' // and no extension method 'ThenBy' .... </code></pre> <p>When I do:</p> <pre><code>var query2 = ((IOrderedQueryable&lt;Plot&gt;)query).ThenBy(p =&gt; p.Number); </code></pre> <p>It compiles, but gives a runtime exception:</p> <blockquote> <p>Expression of type '<code>IQueryable`1[Plot]</code>' cannot be used for parameter of type <code>'IOrderedQueryable`1[Plot]</code>' of method '<code>IOrderedQueryable`1[Plot] ThenBy[Plot,Nullable`1](IOrderedQueryable`1[Plot], Expressions.Expression`1[System.Func`2[Plot,System.Nullable`1[System.Int32]]])</code>'</p> </blockquote> <p>The cast is carried out (I checked), but the parameter of <code>ThenBy</code> is still seen as IQueryable (which puzzles me a bit).</p> <p>Now suppose some method returns an <code>ObjectQuery&lt;Plot&gt;</code> to me as <code>IQueryable&lt;Plot&gt;</code> (like <code>Select()</code>). What if I want to know whether it is safe to call <code>ThenBy</code> on the returned object. How can I figure it out if the <code>ObjectQuery</code> is "real" or a "fake" <code>IOrderedQueryable</code> without catching exeptions?</p>
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    1. USGert Arnold
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    1. COI'd advise against checking the concrete type as it will tie you to your query provider which is exactly what `IQueryable` avoids. One approach is a quick walk of the expression to check for the `SortBy()` method but I suspect somebody will point out something easier. If not, comment on here and I'll provide you the code to walk the tree. Good luck
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      1. USSmudge202
    2. CO@Smudge202 Thanks. There are a lot places much more fun to walk than expression trees, but if you can't resist... But yes, maybe someone else.
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    3. CO@Smudge202 btw, checking the concrete type wouldn't work anyway because it implements both interfaces. It puzzles me that ordinary reflection does not help here, so probably a deeper inspection as you suggest is the only way if I really need to know.
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