>>>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

I got a solution here https://social.msdn.microsoft.com/Forums/en-US/a966a12d-a3c4-40bb-8db8-58ac974b4a52/new-class-using-generics?forum=csharpgeneral

So, the solution was to have base of two classes and then have my other base based on that.