What Are SOLID Principles?
5/21/2025
Hey, everyone! It’s Grant. Today, I want to dig into a topic that’s gaining traction in software development and for good reason—SOLID principles. These five design guidelines can help you write cleaner, more flexible, and maintainable code. Whether you’re just getting started or looking to refine your architectural chops, SOLID is a powerful foundation to master.
What Is SOLID?
SOLID is an acronym for five principles that guide object-oriented programming and design. Let’s walk through each one with simple examples.
1. Single Responsibility Principle
Every class should have one, and only one, reason to change.
In other words, each class should do one job only. If a class is handling both generating reports and saving them to disk, it’s taking on multiple responsibilities. Instead, split those tasks into two separate classes—one for report generation and another for saving.
2. Open/Closed Principle
Software should be open for extension but closed for modification.
This means your code should allow new functionality to be added without changing existing code. For instance, if you’re applying different discount strategies to orders, you can create new discount types through inheritance or interfaces rather than editing the original logic.
3. Liskov Substitution Principle
Subclasses should be replaceable for their parent class without breaking functionality.
Let’s say you have a Bird class with a Fly method. If you create a subclass called Ostrich, which can’t fly, but still inherits that Fly method, it breaks this principle. The solution? Don’t force a subclass to implement behavior it can’t support—restructure the class hierarchy instead.
4. Interface Segregation Principle
Don’t force classes to implement interfaces they don’t use.
Imagine an all-in-one machine interface with Print, Scan, and Fax methods. If a class only needs to print, it shouldn’t be forced to implement Scan and Fax. The better solution is to create smaller, more focused interfaces—like a Printer interface and a Scanner interface—so each class only handles what it needs.
5. Dependency Inversion Principle
High-level modules should not depend on low-level modules. Both should depend on abstractions.
Instead of having your NotificationService directly create an EmailSender, you create a generic MessageSender interface. Now your service just relies on that interface, which makes the code easier to test, swap out implementations, and follow the Dependency Injection pattern.
Why This Matters
SOLID principles help you design systems that are easier to change and scale over time. They make your codebase more understandable, testable, and adaptable to new features and requirements.
Even better? These principles apply across many languages—whether you’re using C#, Java, JavaScript, or Python. They’re not tied to syntax—they’re rooted in smart architecture.
Thanks for reading!
If you’re applying these in your projects or want deeper dives on each principle with visuals or project examples, I’d love to hear about it. Keep building, keep learning.
— Grant Watson
Developer | Creator | Eternal Learner