We can detect the need to use this principle when a change in the module outcomes in a waterfall of changes that affect dependent modules. The design is said to be too inflexible.
The Open/Closed principle (OCP) advises us that we should refactor the application in a manner that future changes don't provoke further modifications.
The form to apply this principle correctly would be by extending the functionality with new code (for instance, using polymorphism) and never changing the old code, which is working already. We can find several strategies to achieve this goal.
Observe that closed for modification is especially meaningful when you have distinct, separate modules (DLLs, EXEs, and so on) that depend on the module to be changed.
On the other hand, using extension methods or polymorphic techniques allows us to perform changes in code without affecting the rest. Think, for example, about the extension methods available in the C# language since version 3.0. You can consider...