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!