Open-Closed Principle

2023-05-30

The Open-Closed Principle is very important to follow when writing Clean Code. Uncle Bob refers to the Open-Closed Principle as the moral center of software architecture. The "Open-Closed" part of the name is saying that in software, modules should be open for extension but closed for modification.

But what do these phrases mean more specifically? "Open for extension" means that code should be organized in such a way that new features can simply be plugged in. "Closed for modification" means that the implementation of these plug-ins should not modify existing source-code. Essentially, this means that when adding a new feature, code should be added instead of changed.

To adhere to the Open-Closed Principle, one must abstract and invert. Ugly switch statements or if-chains should be converted to a single polymorphic method call. Dependencies should point to interfaces to allow modules to be open for extension.

But know that adhering to the Open-Closed Principle perfectly is impossible. Customers will always find a way to want a feature that requires modification. And you should only try to predict changes that are similar to past changes.