public class ClientService : IClientService {
    ...
}

public class ClientService : IClientService {

    private ContextDB context;
    private ClientRepository clientRepo;
    private CityRepository cityRepo;

    //конструктор
    public ClientService{
        context = new ContextDB();
        clientRepo = new ClientRepository(context);
        clientDetailRepo = new ClientDetailRepository(context);
    }

    //метод
    public void MethodSave(Client client){
        using(TransactionScope tran = new TransactionScope()){
            clientRepo.Update(client);
            clientDetailRepo.SomeMethod();
            context.Save();
            tran.Complete();
        }
    }
}