So the call would be:
var result=repository.GetAssociate(new UserName(DependencyContainer.GetAssociate()));
Which you can't call from the persistence layer because
- The Dependency container is defined in an assembly not referenced by the persistence layer
- The UserName constructor is internal to the domain assembly
You could of course messy up your code by using dependency injection on everything that needs testing. This concern does partly fall down if you centralize the coupling between your domain and external dependencies, but remains when you try to test individual methods inside a class, or anything internal, private, or that accesses static methods(most testing frameworks can not handle mocking out static methods, constructors, or factories, as I understand it TypeMock can)
You could make your code messier and more prone to issues in dependent code by changing username to a public interface and changing the persistence layer to accept that interface, but then automatic business logic enforcement goes out the window. If an object outside of the domain assembly can not create a copy of a domain object it has no reason to create, then it's tougher to accidentally not pass things through the proper domain classes/methods.
So how do I test this method?