Entity Framework and Interfaces

gistfile1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Entity;

namespace ConsoleApplication5
{
    public interface IFoo
    {
        string Name { get; }
    }

    public class Foo : IFoo
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }

    public class MyContext : DbContext
    {
        public MyContext() : base("server=.;database=blah;Integrated Security=True")
        {
        }

        public DbSet<Foo> Foos { get; set; }
    }

    public class ThingsWithNames<T> where T : IFoo
    // THIS FIXES IT
    // public class ThingsWithNames<T> where T : class, IFoo
    {
        public IEnumerable<T> Get(IQueryable<T> items)
        {
            return items.Where(i => i.Name != null);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var ctx = new MyContext();

            // this works fine.
            Func<IQueryable<IFoo>, IEnumerable<IFoo>> filter = items => items.Where(i => i.Name != null);
            var foos1 = filter(ctx.Foos).ToList();

            // This throws:
            // Unable to cast the type 'ConsoleApplication5.Foo' to type 'ConsoleApplication5.IFoo'. 
            // LINQ to Entities only supports casting Entity Data Model primitive types.
            var foos2 = new ThingsWithNames<Foo>().Get(ctx.Foos).ToList();
        }
    }
}

Alternatively, you can download my project on Codeplex. I have a working example that you can copy.
https://entityinterfacegenerator.codeplex.com/

It generates the interface files that you need for IoC and unit testing purposes.