Linq Entity Framework generic filter method

Question

I have some methods which perform a standard filter on data from my Entities (using Entity Framework v4).

Example #1:

protected IQueryable

        

Answer 1

I wanted to do the same thing with IQueryable<T> as an input parameter type for the generic method.

I got the runtime exception of "Unable to cast the type.... LINQ to Entities only supports casting Entity Data Model primitive types".

After some time I recognized, I forgot to add the class constraint for the generic parameter. However IQueryable<T> does not require to have the class constraint, it still needs that to resolve the types at runtime.

It would have taken me hours, without this post. Thanks :)

Answer 2

I'd say adding interfaces is a simpler option than messing with the expression tree or reflection. EF entities are partial classes, so you should be able to do something like:

Updated to include class constraint (see Mark's comment)

public interface IHideable
{
  string State { get; }
  string Hidden { get; }
}

...

namespace Database
{
  public partial class Product : IHideable { }
  public partial class Customer : IHideable { }
}

...

protected IQueryable<T> GetActive<T>(ObjectSet<T> entities)
    where T : class, IHideable
{
  var allowedStates = new string[] { "Active" , "Pending" };    

  return (    
    from obj in entities
    where allowedStates.Contains(obj.State)
        && obj.Hidden == "No"
    select obj
  );
}