MostlyScottish.cs

using System;
using System.Collections.Generic;

namespace MostlyScottish {

    //This is what I think we should move towards
    public interface IRepository<T> {
        void Save(T entity);
    }

    
    public class StuffRepository : IRepository<Stuff> {

        public void Save(Stuff entity) {
            //save the Stuff
        }
    }

    public class Stuff {
        //poco DB entity class
    }

    public class StuffDoer {

         private readonly IRepository<Stuff> _StuffRepo;
        public StuffDoer(IRepository<Stuff> stuffRepo) {
            _StuffRepo = stuffRepo;            
        }


        public void MethodUnderTest() {

            // blah blah blah, stuff we can't mock because they are all private instance methods

            IEnumerable<Stuff> stuffCollection = new List<Stuff>();

            foreach (var thing in stuffCollection) {
                _StuffRepo.Save(thing); 
                //Now there is a direct link to the Save method on the StuffRepository.
            }
        }
    }
}

NotScottish.cs

using System;
using System.Collections.Generic;

namespace NotScottish {
    //http://www.youtube.com/watch?v=KrQxQPG8JdE&feature=related


    //So this is what we have now.
    public interface IRepository<T> {
        void Save(T entity);
    }

    public interface IStuffRepository : IRepository<Stuff> {

    }

    public abstract class BaseRepository<T> : IRepository<T> {

        public void Save(T entity) {
            //method that accepts string,Func<ISession,T>
        }
    }

    public class StuffRepository : BaseRepository<Stuff>, IStuffRepository {
        //Trying to use NSubstitute to mock the "Save" method, but where is it really at runtime?
    }

    public class Stuff {
        //poco DB entity class
    }

    public class StuffDoer {

         private readonly IStuffRepository _StuffRepo;
        public StuffDoer(IStuffRepository stuffRepo) {
            _StuffRepo = stuffRepo;            
        }


        public void MethodUnderTest() {

            // blah blah blah, stuff we can't mock because they are all private instance methods

            IEnumerable<Stuff> stuffCollection = new List<Stuff>();

            foreach (var thing in stuffCollection) {
                _StuffRepo.Save(thing); 
                //Where is the Save method located at runtime and what can we really mock at this point?
                //I think it's really located in BaseRepository BUT my magic mocker thinks that it is located on the instance of
                //StuffRepository. So when it mocks "Save" on the StuffRepository, the mock method is located at one place on 
                //the heap, and the real Save method on the BaseRepository is located at another location on the heap. Both the 
                //StuffRepository and BaseRepository have their own MethodTables.
            }
        }
    }
}

Oh in that case ignore my fork.