I have an MVC2 n-tier application (DAL, Domain, Service, MVC web) using a DDD approach (Domain Driven Design), having a Domain Model with repositories. My service layer uses a Request/Response pattern, in which the Request and Response objects contain DTO's (Data Transfer Objects) to marshal data from one layer to the next, and the mapping is done via help from AutoMapper. My question is this: what shape should a DTO typically take? Can it have nested/complex DTO's as well or should it strictly be a flat projection? Or possibly a mixture of both? Also, what are the main reasons for having a flat DTO vs a more complex/nested DTO?
For instance, suppose I had a domain such as the following:
public class Employee
{
public string FirstName { get; set; }
public string LastName { get; set; }
public Company Company { get; set; }
}
public class Company
{
public string Name { get; set; }
public string Address { get; set; }
public string City { get; set; }
public string State { get; set; }
}
There are three different ways I've thought of modeling the Response object.
Option 1 - the DRYest option:
public class GetEmployeeResponse
{
public class EmployeeDTO { get; set; } // contains a CompanyDTO property
}
From the research I've done, it would be inappropriate for a DTO to take a similar shape as the domain object(s) as demonstrated above.
Option 2 - a flattened projection of the domain (anti-DRY):
public class GetEmployeeResponse
{
public string FirstName { get; set; }
public string LastName { get; set; }
public string CompanyName { get; set; }
public string CompanyAddress { get; set; }
public string CompanyCity { get; set; }
public string CompanyState { get; set; }
}
This is more simple, like a DTO apparently should be, but ultimately makes for more DTOs.
Option 3 - a mixture of both:
public class GetEmployeeResponse
{
public EmployeeDTO Employee { get; set; }
public CompanyDTO Company { get; set; }
}
This allows for the code to be a little bit more dry, reusable and manageable, and doesn't expose my domain structure to the end user. The other main benefit is that other responses, like GetCompanyResponse could simply return CompanyDTO, without having to make a copy of all those properties, similar to option 2. What do you think? Which option of these (if any) have you taken and/or have worked for you? If these Request/Responses later get exposed as WCF service methods, does your answer change?
