One truth about business software applications is that they evolve together with business requirements. From different reasons, sometimes from lack of experience of developers and sometimes from not having enough time to devise good solutions, the code often tends to create some sort of cancer inside, which progressively grows to unmanageable pieces of software until it breaks the whole system.
What do we do if we find such phenomenon in your code?
In my experience I have seen that dealing with such code is almost inevitable in brown field projects. Ignoring them makes things only worse. You have to face it and treat it as it is a disease which needs to be cured. Whenever I face such a situation, I follow these steps:
Changing the code without breaking the whole system, or refactoring as is called in software development terminology, is not an easy task. Every change must be validated and tested well to ensure that changes do not break or alter the current functionality. In order to stop the bad code growing, we need to isolate it first. Isolation could be done by aiming abstraction through loose coupling the part which needs to be isolated. The abstraction is done by creating interfaces for every class that needs to be isolated and make dependent classes call to interfaces instead of real classes. This may require introduction of inversion of control (IoC) / Dependency Injection (DI) container. Each isolated part must be supported by quality unit tests which ensure that the abstraction maintains the code logic which was in place. Unit tests will also help us in the second phase.
This phase is quite simple. When you arrive here you have isolated the bad code and protect the functionality with unit tests. Now you can safely start to refactor and change the bad code. As you change, you should continuously run the unit tests to check if changes have any unexpected effects. If the unit test coverage is OK and they are written well, then you can safely proceed as unit tests will spot if you have broken something unintentionally. I would highly recommend Test Driven Development in this phase.
Integrating the isolated code is the last phase, and most probably the easiest. Now the bad code should be gone and the functionality is unchanged. Often you shall find this phase unnecessary as the integration has been done during the phase 2. However, sometimes you will find some kind of double abstractions resulting from a lot of refactorings and you will see the opportunity of cutting these unnecessary code.
Perhaps this strategy looks very simple, but in my experience, it has been a very effective one. We are not always lucky to land in green field projects and define good code from the beginning. Besides, it is not always easy to maintain the code in good shape when the team has many developers and there is not a good development process in place. Sometimes you deliberately allow the code to go in bad direction as a compromise to speed or resources.