>Not absolutely sure I understand your requirements but something like ? :

public class RepositoryBase<T> where T:class
>    {
>        private List<T> _list;
>        protected RepositoryBase(List<T> x )
>        {
>            _list = x;
>        }
>    }
>
>    public class GuestBase<T,TList> : RepositoryBase<T> where T:class where TList:class
>    {
>        protected GuestBase(List<T> thing) : base(thing)
>        {}
>    }
>
>    public class GuestRepository : GuestBase<X,Guests>
>    {
>        private List<Guests> _guests; 
>        public GuestRepository(List<X> xx, List<Guests> guests ) : base(xx)
>        {
>            _guests = guests;
>        }
>    }

I'll see if I can go from here. What about the interface? Our current baseRepository is also using the interface, e.g.

public abstract class RepositoryBase<T> : IRepository<T> where T : class

What should I do with this?