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POIQueryable extension method for linq2entities
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  1. POIQueryable extension method for linq2entities
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    copied!<p>I am trying to implement an extension method that will work with linq2entities. I had initially assumed that if my extension method took and returned an IQueryable, and as long as my expression only made use of supported methods, then it would work fine. I was having alot of trouble, so as a last resort I copied an existing .NET extension method that I knew to work(FirstOrDefault) and simply renamed it. It would seem like it would evaluate the "cannot be translated into a store expression" validation based on the expression returned from the method, not the name of the method itself.</p> <pre><code>var prs = db.People.Where(p =&gt; p.PersonKey == 15).Select(p =&gt; new { id = p.PersonKey, name1 = p.PersonHistories.AsQueryable().AsOf().Name } ).ToList(); </code></pre> <p>My extension method, which is just a copy of FirstOrDefault that I renamed:</p> <pre><code>public static TSource AsOf&lt;TSource&gt;(this IQueryable&lt;TSource&gt; source) { return source.Provider.Execute&lt;TSource&gt;(Expression.Call(null, ((MethodInfo)MethodBase.GetCurrentMethod()).MakeGenericMethod(new Type[] { typeof(TSource) }), new Expression[] { source.Expression })); } </code></pre> <p>Error:</p> <pre><code>LINQ to Entities does not recognize the method 'Models.PersonHistory AsOf[PersonHistory](System.Linq.IQueryable`1[Models.PersonHistory])' method, and this method cannot be translated into a store expression. </code></pre> <p>How do I implement an IQueryable extension method that is supported in Linq2Entities?</p> <p>What I really want AsOf(source, DateTime asOf) to do is soemthing like <code>source.FirstOrDefault&lt;IHistory&gt;(s =&gt; s.EndDate &gt; asOf &amp;&amp; asOf &gt;= s.StartDate )</code>, but I'm not sure how to accomplish this so that it is supported by linq2entities.</p> <p><strong>LINQKIT:</strong> This is what I've come up with using linqkit and I'm hoping somehow I can factor this into something more reusable:</p> <pre><code>Expression&lt;Func&lt;PersonHistory, bool&gt;&gt; IsCurrent = (p) =&gt; p.Ends &gt; DateTime.Now &amp;&amp; p.Starts &lt;= DateTime.Now; var query = db.PersonHistories.Where(IsCurrent); </code></pre> <ol> <li>Have a more globally declared expression, instead of a local variable.</li> <li>Add a DateTime asOf parameter, instead of having .Now hard coded.</li> <li>And if possible, adapt it into an extension method(this sort of is the same as #1, just that an extension method is ideal.</li> </ol>
 

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