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POHow to create an expression at runtime for use in GroupBy() with Entity Framework?
 

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  1. POHow to create an expression at runtime for use in GroupBy() with Entity Framework?
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    <p>I am building a new extension method which will dynamically be able to group query results from Entity Framework.</p> <p>I have been able to build dynamic "where" expressions using LinqKit, but this seems to be a different animal.</p> <p>Intended usage of new extension method:</p> <pre><code>var results = entities.GroupBy("someFieldName").ToList(); </code></pre> <p>Extension method definition:</p> <pre><code> public static IQueryable&lt;IGrouping&lt;object, TEntity&gt;&gt; GroupBy&lt;TEntity&gt;( this IQueryable&lt;TEntity&gt; source, string fieldName) where TEntity : class, IDataEntity { if (string.IsNullOrEmpty(fieldName)) { return new List&lt;IGrouping&lt;object, TEntity&gt;&gt;().AsQueryable(); } var parameter = Expression.Parameter(typeof(TEntity), "x"); var fieldXExpression = Expression.Property(parameter, fieldName); var lambda = Expression.Lambda&lt;Func&lt;TEntity, object&gt;&gt;( Expression.Convert(fieldXExpression, typeof(object)), // throws error when using EF parameter); return source.AsExpandable().GroupBy(lambda); } </code></pre> <p>I need to use <code>Expression.Convert(...)</code> because when I was testing using linqTOobjects, the code failed when the column was a <code>int32</code>. So I needed to manually convert the column to object and it worked great.</p> <p>Now that I'm testing it with EF entities, I guess that EF is trying to translate the convert to equivalent SQL, which of course I know doesn't exist.</p> <p>The error:</p> <pre><code>System.NotSupportedException: Unable to cast the type 'System.String' to type 'System.Object'. LINQ to Entities only supports casting EDM primitive or enumeration types. </code></pre> <p>Is there a way to generate an expression suitable for GroupBy at runtime that is also compatible with EF?</p> <p>Thanks for any guidance.</p>
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    1. COThe Linq to Entities query provider needs to know the type of the property by which to group; you need to add a generic type parameter to the method call, which you will have to specify explicitly and would probably defeat the purpose of using the property name in the first place.
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