Entity Framework: LINQ to Entities only supports casting Entity Data Model primitive types

entity-framework, primitive-types

Solution

The Entity Framework makes this difficult and I'm not sure there's a way to do what you want to do with a single return value type (IComparable, object, etc). You might consider reworking your design into a dictionary of name-to-`Func<IQueryable<K>, IOrderedQueryable<K>>` values:

_possibleSortForForumItem.Add("CreateDate", 
    query => query.OrderBy(item.CreateDate));

And then applying it like so:

var orderedQuery = query.OrderBy(item => item.DefaultOrderColumn);

Func<IQueryable<K>, IOrderedQueryable<K>> assignOrderBy = null;

if (_possibleSortForForumItem.TryGetValue(orderColumnName, out assignOrderBy))
{
    orderedQuery = assignOrderBy(query);
}

Problem

I wrote a method to allow for an Expression to be passed in for the orderby clause, but I ran into this problem. Unable to cast the type 'System.DateTime' to type 'System.IComparable'. LINQ to Entities only supports casting Entity Data Model primitive types. Basically the expression is this: ``` Expression<Func<K, IComparable>> orderBy ``` And is used like this: ``` SomeEntities.SomeTable .Where ( whereClause ) .Select ( selectClause ) .OrderBy(orderBy) ``` The idea is so that I can use a dictionary to hold string matches to expressions like: ``` _possibleSortForForumItem.Add("CreateDate", item => item.CreateDate); ``` Then I have a method that takes in the sort string and returns the expression if it matches a key in the dictionary, if not returns some default. (The idea being a way to control what it can be ordered by) Now this works for String properties, but so far not for datetime or integer as I get the error message above. Now far as I (loosely) understand the problem is that Entity Framework needs it to be a Primary/EDM type because it has to convert the C# DateTime into something the database can handle. Is there a way to convert the datetime to a primitive type so that this will still work? Solution The method for getting the order by method: (Take in a query and return it in "ordered form") ``` private static Func<IQueryable<ForumViewItem>, IOrderedQueryable<ForumViewItem>> GetMethodForSort(String sortBy) { if (_methodForSort == null) { _methodForSort = new Dictionary<String, Func<IQueryable<ForumViewItem>, IOrderedQueryable<ForumViewItem>>>(); _methodForSort.Add(SortForumViewItemCreatedOn, item => item.OrderBy(innerItem => innerItem.CreatedOn)); ... } Func<IQueryable<ForumViewItem>, IOrderedQueryable<ForumViewItem>> orderMethod; if(String.IsNullOrEmpty(sortBy) || !_methodForSort.ContainsKey(sortBy)) { orderMethod = _methodForSort["ForumName"]; } else { orderMethod = _methodForSort[sortBy]; } return orderMethod; } ``` The method signature for the generic query method: ``` IList<K> GetListForGrid<T, K>(this ObjectQuery<T> query, ... Func<IQueryable<K>, IOrderedQueryable<K>> orderBy, ...) ``` And the use of the passed in method: ``` initialQuery = query .Where ( somethingEqualsSomething ) .Select ( selectClause ); var orderedQuery = orderBy(initialQuery); returnValue = orderedQuery .Skip(numberToShow * realPage) .Take(numberToShow) .ToList(); ```

Original source