SOLID in .NET: 5 principles that make your code clearer and more maintainable
We hear a lot about the SOLID principles. Between overly theoretical definitions and overly complex examples, the simplest way in is probably to describe anti-SOLID code first.
Anti-SOLID code
A 5,000-line MegaManager class doing anything and everything, with 600-line methods handling invoicing, HR and the weather. No interface contracts, hard-coded dependencies, and an inheritance tree that looks like the Kardashian family tree. And every change breaks the app in 25 places. That's it.
The 5 principles
Single Responsibility: structure your classes so they stay easy to read and test. One class = one clear business responsibility. I wrote a dedicated article on this first principle, with a concrete refactoring example.
Open/Closed: add new cases without breaking what exists. Your code is open to extension but closed to modification. You add features without touching existing code.
Liskov Substitution: build consistent inheritance. A derived object must be usable in place of its parent without breaking behavior. A square is a rectangle, but not every rectangle is a square.
Interface Segregation: avoid bloated interfaces. Several small interfaces beat forcing a class to implement methods that don't match its role. Create targeted, specific interfaces.
Dependency Inversion: avoid hard-wired dependencies. Your code relies on interfaces to stay flexible and easily testable. Depend on abstractions, not implementations.
Guidelines, not dogma
These principles are not absolute rules. They are not "patterns to apply at all costs".
Takeaway: they are simple guideposts for writing cleaner, more stable, testable code that stays easy to evolve in a real project, over the long run.
This article comes from one of my LinkedIn posts (in French): join the discussion ↗