C# Interfaces implementation

Question

I don't know how manage properly the interfaces in C#. My goal is to have an abstract class for my Business Layer Services that have some common methods (like Save(), Dispose()), that call different DAL repository methods. I wish to avoid to repeat in all my services something like:

public Save() 
{
   repository.Save();
}

I have a scenario similar to that:

Interface

namespace Common
{
    public interface IRepository
    {
        void Save;
        void Dispose;
    }
}

DAL

namespace DAL
{
    public Repository : IRepository
    {
        public void Save() {};
        public void Dispose() {};
        public void Add() {}
    }
}

BL

namespace BL
{
    public abstrac BaseService
    {
        protected IRepository repository;
        protected BaseService(IRepository repo)
        {
            repository = repo;
        }

        public Save() 
        {
            repository.Save();
        }
    }

    //...

    //Tentative 1
    public Service : BaseService
    {
        private Repository rep;

        public Service()
            : base(new DAL.Repository())
        {
            rep = base.repository; // ERROR: cannot convert IRepository to Repository
        }
    }
}

I tried also this:

//Tentative 2
public Service : BaseService
{
    private IRepository rep;

    public Service()
        : base(new DAL.Repository())
    {
        rep = base.repository; // OK
    }

    public void Add()
    {
        rep.Add() // ERROR: IRepository doesn't contain a definition for 'Add'
    }
}

I know I could define in the interface all the methods I want to use, but I'll will have to manage a lot of problems with generic types and, as you should have understand from my question, I'm quite new in C# and I wish to avoid complexity is is possible, utill I'll be more expert at least :)

Answer 1

public Service()
        : base(new DAL.Repository())
    {
           rep = (Repository)base.repository;

    }

This way u will get the Add() service which is not a part of IRepository but a newer implementation in the extended class.