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