设计模式混乱

Question

I am brushing up on my design patterns at the moment and got a little confused when I came across this tutorial:

http://www.asp.net/mvc/tutorials/iteration-4-make-the-application-loosely-coupled-cs

If you look at listing 7 onwards, the author says it is using the decorator pattern. However, is one of the main principles of this pattern to wrap objects and ADD responsibilities and behaviour?

I think it looks more like and adapter pattern as it is adapting the MVC specific ModelStateDictionary to work with a more flexible IValidationDictionary so that different implementations can be used with the service if WPF etc were used instead. There is new responsibility or behaviour added.

Do I have this correct or not? If I'm wrong can anyone please explain why?

Thanks

Answer 1

I think you're correct, and that there's an error in the post. From the article:

The Decorator pattern enables you to wrap an existing class in a new class in order to implement an interface.

That's not exactly true - decorators do allow you to wrap one implementation inside another, but the intention usually isn't to implement another interface, but to "decorate" the instance with new functionality. The adapter pattern allows you to take two dissimilar interfaces, and modify one instance to be have like another.

Answer 2

I agree with you, that looks to me like the Adapter Pattern , that is, the ModelStateDictionary is abstracted behind the interface IValidationDictionary (the adapter interface) using a concrete type (the adapter) such that the implementation can be changed later.

The Decorator Pattern usually provides additional functionality via composition, exposing the same interface as the decorated type. This is usually done via sub-classing or through interface implementation.

An example of a decorator would be:

1. you have a repository class that fetches "objects" from the database
2. you have a repository decorator class that can cache objects without needing to fetch them from the database each time. This decorator class provides the cache fetching and retrieving through composition by sub-classing the original repository class and overriding the Get() method to first check the cache for the item (and Save() would be overridden to also update the cache as well as the database).