EF/LINQ: Where() against a property of a subtype
- by ladenedge
I have a set of POCOs, all of which implement the following simple interface:
interface IIdObject
{
int Id { get; set; }
}
A subset of these POCOs implement this additional interface:
interface IDeletableObject : IIdObject
{
bool IsDeleted { get; set; }
}
I have a repository hierarchy that looks something like this:
IRepository<T <: BasicRepository<T <: ValidatingRepository<T (where T is IIdObject)
I'm trying to add a FilteringRepository to the hierarchy such that all of the POCOs that implement IDeletableObject have a Where(p => p.IsDeleted == false) filter applied before any other queries take place. My goal is to avoid duplicating the hierarchy solely for IDeletableObjects.
My first attempt looked like this:
public override IQueryable<T> Query()
{
return base.Query().Where(t => ((IDeletableObject)t).IsDeleted == false);
}
This works well with LINQ to Objects, but when I switch to an EF backend I get: "LINQ to Entities only supports casting Entity Data Model primitive types."
I went on to try some fancier parameterized solutions, but they ultimately failed because I couldn't make T covariant in the following case for some reason I don't quite understand:
interface IQueryFilter<out T> // error
{
Expression<Func<T, bool>> GetFilter();
}
I'd be happy to go into more detail on my more complicated solutions if it would help, but I think I'll stop here for now in hope that someone might have an idea for me to try.
Thanks very much in advance!
