SOLID Principles

TL;DR: SRP means each class or module should have only one reason to change. Split validation, data access, and business logic into separate classes. SRP improves maintainability, testability, and scalability. Use C# 12 features like primary constructors and records for clean separation. Avoid “God” classes and mixing unrelated responsibilities. SRP is the foundation for applying other SOLID principles. Refactor large classes by extracting focused components and using dependency injection. The Single Responsibility Principle gets misunderstood more than any other SOLID principle. It’s not about doing one thing, it’s about having one reason to change. When your class changes for multiple business reasons, you’ve violated SRP and created a maintenance nightmare. ...

TL;DR: The Open/Closed Principle (OCP) states: classes should be open for extension, but closed for modification. You don’t need to create a plugin for every new feature, favor simpler patterns like strategy, inheritance, or composition. Start with well-named abstractions; introduce extensibility points only when real change is expected. Keep code maintainable by balancing extension points with simplicity. Overengineering is a bigger risk than occasional refactoring. The Open/Closed Principle isn’t about making everything extensible. It simply means that your core business logic should be closed to modification but open to extension. The real skill is knowing when to apply it and when you’re just over-engineering your code. ...

TL;DR LSP means you can use subtype objects anywhere you use base type objects without breaking code. Square/Rectangle inheritance that breaks existing code? That’s a classic LSP problem. Watch for subclasses that throw unexpected exceptions or silently change behavior. When behaviors are too different, reach for composition instead of inheritance. Make interfaces that clearly show what objects can and can’t do to avoid surprises. LSP violations often sneak past unit tests and show up as runtime bugs. The Liskov Substitution Principle isn’t just academic theory - it helps you avoid those “it worked yesterday” bugs that show up in production. When a subclass doesn’t behave like its parent promises, your code breaks in ways that unit tests often miss. ...

TL;DR ISP means interfaces should be small and focused on client needs. Avoid “God” interfaces that force clients to implement unused methods. Split large interfaces into cohesive, role-based interfaces. Use C# 12 features like default interface methods for flexibility. ISP improves maintainability, testability, and reduces coupling. Refactor fat interfaces by extracting related methods into separate interfaces. Interface Segregation Principle stops you from creating huge interfaces that force clients to implement methods they’ll never use. When interfaces get too big, your implementations end up filled with empty methods and unnecessary dependencies. ...

TL;DR DIP means depend on abstractions, not concrete implementations. Use interfaces and dependency injection to decouple business logic from details. DIP improves testability, flexibility, and maintainability in C# code. Avoid leaky abstractions, unnecessary interfaces, and service locator anti-patterns. Use C# 12 primary constructors and .NET 8 DI features for clean, modern architecture. The Dependency Inversion Principle helps you turn rigid, tightly-coupled code into flexible, testable systems. Rather than depending on concrete implementations, your high-level modules rely on abstractions. This goes beyond dependency injection, it’s about changing the direction of control flow. ...