How do you use factory template interfaces in a domain-driven design?

Question

Does it make sense to use interfaces for your default domain object factories, or should interfaces be reserved for factory classes only when you need them?

public IUserFactory
{
    User CreateNewUser();
}

public UserFactory : IUserFactory
{
    public User CreateNewUser()
    {
        return new User();
    }
}

Answer 1

Here's a translation of the same problem in Java.

Source example

public interface UserFactoryIF
{
   User createNewUser();
}

Then Factory implementation

public class UserFactory implements UserFactoryIF
{
     public User createNewUser()
     {
         // whatever special logic it takes to make a User
         // below is simplification
         return new User();
     }
}

factory, . , , factory . , :

public interface User
{
    public String getName();
    public long getId();
    public long getUUID();
    // more biz methods on the User
}

factory :

public class UserFactory {

    public static User createUserFrom(Person person) {
        // ...
        return new UserImpl( ... );
    }

    public static user createUserFrom(AmazonUser amazonUser) {
         // ... special logic for AmazonWS user
        return new UserImpl( ... );          
    }

    private static class UserImpl implements User {
       // encapsulated impl with validation semantics to
       // insure no one else in the production code can impl
       // this naively with side effects
    }
}

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Answer 2

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Answer 3

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Answer 4

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Answer 5

Two things: (1) I will wait until I need (or see an impending need) an alternative implementation before I create the interface, and (2) the interfaces almost always simplify unit testing, especially with ridicule, so I often come up with the need for an interface.