C＃中的类设计结构

Question

Entity Class:

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

public class Invoice {
        public int InvoiceId { get; set; }
        public int CustomerId { get; set; }
        public string InvoiceNo { get; set; }
}

Interface:

public interface ICustomerService {
    Customer GetCustomerById(int customerId);
    void DeleteCustomer(int customerId);
}

public interface IInvoiceService {
    Invoice GetInvoiceById(int invoiceId);
    void DeleteInvoice(int invoiceId);
    List<Invoice> GetAllInvoiceByCustomer(int customerId);
    Customer GetInvoiceCustomer(int invoiceId);
}

Class:

public class CustomerService : ICustomerService {

    private readonly IInvoiceService _invoiceService = new InvoiceService();

    public Customer GetCustomerById(int customerId) {
        //return customer from db
        return new Customer();
    }

    public void DeleteCustomer(int customerId) {
        var invoiceList = _invoiceService.GetAllInvoiceByCustomer(customerId);
        foreach (var invoice in invoiceList) {
            _invoiceService.DeleteInvoice(invoice.InvoiceId);
        }

        //delete customer from db

    }
}

public class InvoiceService : IInvoiceService {

    private readonly ICustomerService _customerService = new CustomerService();

    public Invoice GetInvoiceById(int invoiceId) {
        //return invoice from db
        return new Invoice();
    }

    public void DeleteInvoice(int invoiceId) {
        //delete invoice from db
    }

    public List<Invoice> GetAllInvoiceByCustomer(int customerId) {
        //get all invoice by customer id
        return new List<Invoice>();
    }

    public Customer GetInvoiceCustomer(int invoiceId) {
        Invoice invoice = GetInvoiceById(invoiceId);
        return _customerService.GetCustomerById(invoice.CustomerId);
    }

}

When I create a new instance for "CustomerService". It will return an error:

An unhandled exception of type 'System.StackOverflowException' occurred

Because when I create new instance for "CustomerService", "CustomerService" will create a new instance for "InvoiceService", "InvoiceServer" also create a new instance of "CustomerServer".

1) Should I set all the methods to static?

2) "InvoiceService" will have call methods from "CustomerService", and "CustomerService" will call methods from "InvoiceSercie" too. How can I write the classes? If I set all the method to static, problem will be fixed, but I guess it is not a good solution.

Thank you very much!

Answer 1

You have to choose one of the classes to pass itself as a reference to the other. Let's say it's customer service:

public class CustomerService : ICustomerService {

  private readonly IInvoiceService _invoiceService = new InvoiceService(this);

  ...
}

public class InvoiceService : IInvoiceService {

  private readonly ICustomerService _customerService;

  public class InvoiceService(ICustomerService customerService) {
    _customerService = customerService;
  }

}

Now the cycle is broken...

Another option is to use a dependency injection framework, like StructureMap or Ninject.

Answer 2

In general, I would recommend having less coupling between the classes. Each class should do one thing (Customer and Invoice) and then create a third class which uses both. For example, you could create a class called "CustomerInvoicer" which takes the two interfaces in its constructor and move the method "GetInvoiceCustomer" to that new class. In my experience, that will make it a lot more maintainable in the long run, as each class has a single responsibility, and your ultimate consumer only needs to use the one main class (which may have more advanced logic).

public class CustomerInvoicer {

    private readonly ICustomerService _customerService;
    private readonly IInvoiceService _invoiceService;

    public CustomerInvoicer(ICustomerService cust, IInvoiceService inv) {
        _invoiceService = inv;
        _customerService = cust;
    }


    public Customer GetInvoiceCustomer(int invoiceId) {
        Invoice invoice = _invoiceService.GetInvoiceById(invoiceId);
        return _customerService.GetCustomerById(invoice.CustomerId);
    }
}

Also, I would recommend using a Dependency Injection library with this approach.