如何切换具有相同名称和不同名称空间的类？

Question

I have read the blog post Repository Design Pattern

The main idea of the article is to cast classes with same name to get more functionality. It's ok, I understand it. But i can't understand this code:

repository.query(new NewestNewsesSpecification());

He has NewestNewsesSpecification for sql and NewestNewsesSpecification for Realm.

So my question - how the program can understand what implementation of NewestNewsesSpecification to use, if classes have the same name but in different namespaces? It's not even a DI framework. Or is it just that every time I need different storage I need to find and change namespaces in source code files?

The code of article is in Java. I'm writing in C#.

Answer 1

I just read the article very quickly. He is explaining the Repository Pattern. This helps to encapsulate implementation details of the actual used repository. This way you can swap underlying repositories without modifying your client code. The underlying details are encapsulated in the Specification class. You have to explicitly pass an implementation of this base class to the repository eg into the 'query' method. This is the point where you the client have to decide which repository to use (or to query) by creating the appropriate instance.

If you have multiple implementations sharing the same name then they are required to reside in a different namespace to avoid ambiguities. You are right if you assume that the compiler can't resolve this colliding references. Usually you have to fully qualify the type (eg instantiation, cast): 'new Sql.NewestNewsesSpecification()' or use the compiler directive 'using' or 'import' to declare the appropriate namespace so that the short form can be used: 'new NewestNewsesSpecification()'. The article assumes that you include either 'Sql' namespace or the 'Realm' namespace. Main idea was, my guess, to show which parts of code will change when replacing the underlying repository (the context). If you include both namespaces simultaneously, you can specify an alias for one of the implementations or use fully qualified references so that the references are not ambiguous any more.