c# - Application Services Composition

Question

I've a Customer entity that has an referente to City:

public class Customer
{
    public int CustomerId { get; set; }

    public int CityId { get; set;}
}

Then, the following application service:

public class CustomerService
{
    private readonly ICustomerRepository customerRepository;

    public CustomerService(ICustomerRepository customerRepository)
    {
        this.customerRepository = customerRepository;
    }

    public void RegisterCustomer(Customer customer)
    {
        //Do another things...

        customerRepository.Save(customer);
    }
}

To register a new customer the service consumer would have to have access to a listing of cities to put in CityId, such as get the list of cities to fill a combobox.

Thus it would be necessary provide a list city operation.

Should i add this operation to CustomerService?

Like:

public class CustomerService
{
    private readonly ICustomerRepository customerRepository;
    private readonly ICityRepository cityRepository;

    public ServicoCliente(
        ICustomerRepository customerRepository, 
        ICityRepository cityRepository)
    {
        this.customerRepository= customerRepository;
        this.cityRepository= cityRepository;
    }

    public void RegisterCustomer(Customer customer)
    {
        customerRepository.Save(customer);
    }

    public List<City> ListCities()
    {
        return cityRepository.GetAll();
    }
}

Or create a new Service:

public class CityService
{
    private readonly ICityRepository cityRepository;

    public CityService(ICityRepository cityRepository)
    {
        this.cityRepository= cityRepository;
    }

    public List<City> ListCities()
    {
        return cityRepository.GetAll();
    }
}

In the latter case the consumer should have references two services to be able to complete one operation: RegisterCustomer.

Which approach to follow? What are the advantages and disadvantages of this?

Answer 1

This is a code organization issue and has little to do with DDD.

public class CustomerService {

public List ListCities() { return cityRepository.GetAll(); }

}

There's an obvious mismatch here -- you don't expect from a Customer service public interface method to return cities. This breaks the principle of least surprise and will cause a lot of searching and head scratching for generations of developers to come.

A dedicated service seems like a better idea to me.