Question

I have an extension method for a common interface used on my entities

public static IQueryable<T> IsEdited<T>(this IQueryable<T> source)
        where T : IAuditData
{
    return from o in source
           where o.CreatedOn != o.UpdatedOn
           select o;
}

when I call it

var editedUsers = dataContext.Users.IsEdited();

I get

Unable to cast the type ‘IUserData’ to type
‘IAuditData’. LINQ to Entities only
supports casting Entity Data Model primitive types

IUserData is an interface too, I use it to abstract the DbContext’s DbSet properties

IQueryable<IUserData> Users { get { ... } }

but if I add a class constraint, all is well

public static IQueryable<T> IsEdited<T>(this IQueryable<T> source)
        where T : class, IAuditData
{
    return from o in source
           where o.CreatedOn != o.UpdatedOn
           select o;
}

which makes me happy.
But what it is that the compiler is doing to tell EF that T is a reference type?

Answer 1

struct type are not supported by EF. If you do not constraint the parameter as a class it can also be a struct.