SOLID Principles
Five principles for building software that is easy to understand, extend, and maintain. They reduce coupling, increase cohesion, and make code testable.
When to Apply
Reference these principles when:
- Designing new classes, modules, or interfaces
- Refactoring code with too many responsibilities
- Reviewing PRs for architectural concerns
- Breaking apart god objects or fat interfaces
- Deciding where to draw module boundaries
- Making code more testable
Quick Reference
| Principle | One-Liner | Red Flag |
|---|---|---|
| SRP | One reason to change | "This class handles X *and* Y *and* Z" |
| OCP | Add, don't modify | Growing if/else or switch chains for types |
| LSP | Subtypes are substitutable | Type-checking or special-casing in calling code |
| ISP | Small, focused interfaces | Empty method implementations or throw new Error("Not implemented") |
| DIP | Depend on abstractions | new ConcreteClass() inside business logic |
See references/PRINCIPLES.md for detailed explanations and TypeScript examples.
Detection Checklist
Ask these questions for every class and module:
| Question | Violated Principle |
|---|---|
| Does this class have multiple reasons to change? | SRP |
| Do I need to modify existing code to add a new variant? | OCP |
| Does calling code need type-checks or special cases for subtypes? | LSP |
| Are implementors forced to stub out unused methods? | ISP |
| Does high-level logic directly instantiate infrastructure? | DIP |
Applying SOLID at Different Scales
| Scale | SRP | OCP | LSP | ISP | DIP |
|---|---|---|---|---|---|
| Function | Does one thing | — | — | — | Takes abstractions as params |
| Class | One reason to change | Extend via composition | Subtypes honor contracts | Implements only what it uses | Constructor injection |
| Module | One bounded context | Plugin architecture | Interchangeable implementations | Thin public API | Depends inward |
| Service | Single domain | New features = new services | API contract stability | Minimal API surface | Abstractions at boundaries |
Relationships Between Principles
- SRP + ISP: Splitting responsibilities often means splitting interfaces too
- OCP + DIP: Depending on abstractions is what makes extension without modification possible
- LSP + OCP: If subtypes are substitutable, you can extend behavior by adding new subtypes
- DIP + ISP: Small focused interfaces make dependency inversion practical
Common Anti-Patterns
| Anti-Pattern | Violated Principles | Fix |
|---|---|---|
| God class doing everything | SRP | Extract focused classes |
switch on type across codebase | OCP, LSP | Replace with polymorphism |
| Subclass that throws "not supported" | LSP, ISP | Redesign hierarchy, split interface |
| Fat interface with 20 methods | ISP | Split into role-based interfaces |
| Business logic importing DB driver | DIP | Inject repository interface |
| Service creating its own dependencies | DIP | Constructor injection |
Best Practices
DO
- Start with SRP — it's the foundation for all others
- Use interfaces to define boundaries between components
- Let violations emerge from real problems, then fix them
- Prefer composition over inheritance for extending behavior
- Keep interfaces small and role-specific
- Inject dependencies through constructors
DON'T
- Apply SOLID dogmatically to trivial code (a 5-line utility doesn't need an interface)
- Create abstractions before you have at least two implementations
- Confuse SRP with "single method" — it's about reasons to change, not size
- Force Liskov compliance on classes that shouldn't be in the same hierarchy
- Over-segregate interfaces into single-method fragments when a cohesive group makes sense