>>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?

I guess you could use composition (though there may be something better):

public interface IGuestBase<TList> where TList:class
    {
        
    }

// then
 public class GuestRepository : GuestBase<X,Guests>,IGuestBase<Guests>
    {
     }

BTW GuestBase is probably not a good name for that class since it is still generic