Interface implimenttaion of the class issue

Question

Here is the my simple senario. I have a class(BLL class) which implemented an interface. What I wanna do is, inside the presantation layer, i want users reach only the interface and interacting the class with this interface, not the class function directly. Is there anyway to do that?

My BLL class implemeted interface :

    public interface IOfis
    {

        bool Add(Ofis ofs);
        bool Update(Ofis ofs);


    }

    public class BLL_Ofis:IOfis
    {
        public bool Update(Ofis ofs)
        {
            DAL_Ofis ofs_dal = new DAL_Ofis();
            try
            {
                return ofs_dal.Update(ofs);
            }
            catch (Exception)
            {

                throw;
            }
            finally { ofs_dal = null; }
        }

      public bool Add(Ofis ofs_obj)
        {
            DAL_Ofis ofs_dal = new DAL_Ofis();
            try
            {
                return ofs_dal.Add(ofs_obj);
            }
            catch (Exception)
            {

                throw;
            }
            finally { ofs_dal = null; }
        }
}

Inside the presantatoin layer, i use it like this :

IOfis bll_ofis = new BLL_Ofis();
bll_ofis.Update();

But in this situation I can also reach the class directly like this :

 BLL_Ofis bll_ofis = new BLL_Ofis();
 bll_ofis.Update();

which I dont want that. I wanna reach only method which declared inside the Interface.

Answer 1

I don't see why you would want it, but a "solution" could be to change the public methods into explicit interface implementations. To do that, remove access modifiers (here public) and prepend IOfis. to the name of the method. Like this:

public class BLL_Ofis : IOfis
{
    bool IOfis.Update(Ofis ofs)
    {
        ...
    }

    ...
}

An explicit interface implementation can only be invkoked when the compile-time type of the variable is the interface type.