How to refactor tightly coupled classes?

Question

I'm trying to refactor a big tightly coupled application and trying to make it more maintainable and flexible.

I've got many unit tests, so I'm hoping to refactor step by step.

Which Design & Refactoring Patterns should I consider implementing / applying to accomplish this task ?

I can think of some :

• Extract Interface
• Extract Method
• Dependency Injection

Also feel free to share your own experience and best practices for this kind of refactoring job.

UPDATE

I'm carrying out this refactoring because of the reasons explained in this question. Basically I can't implement a plugin system without extracting an couple of interfaces and those interfaces are highly coupled which requires separated the application in 40+ DLLs to just compile without circular referencing problem.

Answer 1

In all seriousness, refactoring is not an undertaking to be taken lightly, especially in tightly coupled systems. It can often seem like a worthwhile task before it has been performed, but from my experience, it can soon become a burden once started as it is often more likely to introduce new bugs than solve any existing issues.

Before embarking on a major refactoring, you should consider what the gains will be and what alternatives there are (such as creating a new product from scratch, refactoring only the parts that need it right away, etc.). You should certainly have a good understanding of the architecture, roles, and responsibilities before starting, as well as the expected and existing behavior to ensure that you know when you've broken something.

Also, it can be beneficial to draw up a design of how it will be after the refactoring and how that maps to the current implementation so that you can stay on point. You should also regression test as often as possible.

It can be frustrating to the perfectionist when a design is quite obviously in need of refactoring, but sometimes one has to consider the cost/benefit of the changes and concede the battle. If you must make the changes, tread carefully and don't try to do too much at once.

Answer 2

This is a big question, and one could write an entire book in answer to it. Fortunately, someone already has. Grab yourself a copy of Working Effectively with Legacy Code by Michael Feathers. It's pretty much an entire book devoted to answering your question.

It's a really good book, too. I'd definitely put it up there with Code Complete, Design Patterns, and Pragmatic Programmer on a list of books that should be in every developer's library.