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POHow do I combine two Member Expression Trees?
 

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  1. POHow do I combine two Member Expression Trees?
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    <p>I'm trying to combine the following expressions into a single expression: item => item.sub, sub => sub.key to become item => item.sub.key. I need to do this so I can create an OrderBy method which takes the item selector separately to the key selector. This can be accomplished using one of the overloads on OrderBy and providing an <code>IComparer&lt;T&gt;</code>, but it won't translate to SQL.</p> <p>Following is a method signature to further clarify what I am trying to achive, along with an implementation that doesn't work, but should illustrate the point.</p> <pre><code> public static IOrderedQueryable&lt;TEntity&gt; OrderBy&lt;TEntity, TSubEntity, TKey&gt;( this IQueryable&lt;TEntity&gt; source, Expression&lt;Func&lt;TEntity, TSubEntity&gt;&gt; selectItem, Expression&lt;Func&lt;TSubEntity, TKey&gt;&gt; selectKey) where TEntity : class where TSubEntity : class { var parameterItem = Expression.Parameter(typeof(TEntity), "item"); ... some magic ... var selector = Expression.Lambda(magic, parameterItem); return (IOrderedQueryable&lt;TEntity&gt;)source.Provider.CreateQuery( Expression.Call(typeof(Queryable), "OrderBy", new Type[] { source.ElementType, selector.Body.Type }, source.Expression, selector )); } </code></pre> <p>which would be called as:</p> <pre><code>.OrderBy(item =&gt; item.Sub, sub =&gt; sub.Key) </code></pre> <p>Is this possible? Is there a better way? The reason I want an OrderBy method that works this way is to support a complex key selection expression that applies to many entities, though they are exposed in different ways. Also, I'm aware of a way to do this using String representations of deep properties, but I'm trying to keep it strongly typed.</p>
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