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POHow to create C# LambdaExpression that returns anonymous type for SelectMany resultSelector
 

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  1. POHow to create C# LambdaExpression that returns anonymous type for SelectMany resultSelector
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    <p>I'm building a dynamic query that can have <strong>n</strong> of <code>Where</code> method calls and <strong>n</strong> of <code>SelectMany</code> calls dependent upon user input. For example I may have:</p> <pre class="lang-cs prettyprint-override"><code>var qZ = entityContext.TableA .SelectMany(a=&gt;a.TableB, (a,t)=&gt;new{a,t} ) .Where(a=&gt;a.t.FieldID==21) .Where(a=&gt; EntityFunctions.Left(a.t.Value,1)=="p") .SelectMany(a=&gt;a.a.TableC, (a,t)=&gt;new{a,t} ) .Where(a=&gt;a.t.FieldID==22) .Where(a=&gt; a.a.t.Value=="Peter" &amp;&amp; a.t.Value=="Pan") .Where(a=&gt; a.a.a.TypeID==3) .Select(a=&gt; new{ a.a.a.ItemID } ).Distinct(); </code></pre> <p>In the method I'm writing, I use helper methods that return an <code>IQueryable</code> as seen in the return line below. </p> <pre class="lang-cs prettyprint-override"><code>return query.Provider.CreateQuery( Expression.Call(typeof(Queryable), "Where", new Type[] {query.ElementType}, query.Expression, predicateLambda) ); </code></pre> <p>I'm able to create LambdaExpressions for all of the various query <em>attribute-value</em> pairs required, but I am unable to create one for the <code>resultSelector</code> of <code>Queryable.SelectMany</code>.</p> <p>How can we create <code>(a,t) =&gt; new{a=a, t=t}</code> in an expression tree? Or How do we accomplish the same result as the .SelectMany above using Expression.Call like below?</p> <pre class="lang-cs prettyprint-override"><code>Expression.Call(typeof(Queryable), "SelectMany", ????????, ???????? ); </code></pre> <p>I've tried using the <code>SelectMany</code> overload that doesn't require the <code>resultSelector</code> which works to some degree, however, I don't know how to reference the properties of t in subsequent method calls.</p> <p>I've found this lambda expression (<code>(a,t) =&gt; new{a=a, t=t}</code>) associated with <code>SelectMany</code> all over the web, but I can't find any example of how to convert it to an expression tree.</p> <p>UPDATE: Let's reframe the question. I can pass the lambda like this</p> <pre class="lang-cs prettyprint-override"><code>var q = entityContext.TableA.AsQueryable(); var q1 = Queryable.SelectMany(q, a =&gt; a.TableB, (a, t) =&gt; new { a = a, t = t }); var q2 = Queryable.Where(q1,a=&gt;a.t.FieldID==22); </code></pre> <p>That works, however, since I don't know ahead of time how many SelectMany need to be called and since each call changes to anonymous type of the IQueriable, is there a way to cast (and re-cast) the anonymous type to a single variable? This way I can loop through and apply whatever method necessary to the variable and then enumerate to get the results once the query is built. Something like:</p> <pre class="lang-cs prettyprint-override"><code>var q = entityContext.TableA..AsQueryable(); q = Queryable.SelectMany(q, a =&gt; a.TableB, (a, t) =&gt; new { a = a, t = t }); q = Queryable.Where(q,a=&gt;a.t.FieldID==22); </code></pre> <p>(BTW: This doesn't work)</p>
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    1. COyou will have to declare the anonymous class statically and pass it to your expression builder (if you manage to do this without declaring it statically, you'll end up with a DataRow like syntax to access properties and iin that case it's easier to use DataTables ^^)
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