SOLID Principles Guide

Overview

SOLID is a set of five object-oriented design principles that promote maintainable, flexible, and scalable code. This skill provides guidance for detecting violations and applying correct patterns across multiple languages.

The Five Principles

S - Single Responsibility Principle (SRP)

A class should have only one reason to change.

Detection Patterns:

• Classes with multiple unrelated methods
• Files exceeding 200-300 lines
• Class names containing "And", "Manager", "Handler" doing too much
• Methods that mix I/O, business logic, and presentation

Refactoring Strategy:

• Extract cohesive functionality into separate classes
• Use composition to combine smaller components
• Apply facade pattern for unified interfaces

O - Open/Closed Principle (OCP)

Software entities should be open for extension but closed for modification.

Detection Patterns:

• Switch statements on type that grow with new types
• Repeated if/else chains checking object types
• Modifications to existing code for new features

Refactoring Strategy:

• Use polymorphism and inheritance
• Apply strategy pattern for varying behaviors
• Implement plugin architectures

L - Liskov Substitution Principle (LSP)

Subtypes must be substitutable for their base types.

Detection Patterns:

• Overridden methods throwing unexpected exceptions
• Subclasses that don't use inherited methods
• Type checks before calling base type methods
• Empty or no-op implementations of inherited methods

Refactoring Strategy:

• Favor composition over inheritance
• Use interface segregation
• Create proper type hierarchies

I - Interface Segregation Principle (ISP)

Clients should not be forced to depend on interfaces they don't use.

Detection Patterns:

• Interfaces with many methods (>5-7)
• Classes implementing interfaces with unused methods
• "Fat" interfaces that try to do everything

Refactoring Strategy:

• Split large interfaces into smaller, focused ones
• Use role interfaces
• Apply interface composition

D - Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules; both should depend on abstractions.

Detection Patterns:

• Direct instantiation of concrete classes
• Hard-coded dependencies
• Import of implementation details in high-level modules

Refactoring Strategy:

• Introduce interfaces/abstractions
• Use dependency injection
• Apply factory patterns

Violation Severity Levels

Severity	Description	Action
Critical	Principle completely ignored, major maintenance issues	Immediate refactoring required
High	Clear violation affecting multiple areas	Schedule refactoring soon
Medium	Partial violation, localized impact	Refactor during related changes
Low	Minor deviation, minimal impact	Note for future improvement

Analysis Workflow

To analyze code for SOLID violations:

1. Scan for SRP violations first - Large files and multi-purpose classes
2. Check inheritance hierarchies - LSP and OCP violations
3. Examine interfaces - ISP violations in interface definitions
4. Trace dependencies - DIP violations in module imports
5. Document findings with severity and refactoring suggestions

Language-Specific Considerations

TypeScript/JavaScript

Focus on module boundaries, class size, and interface definitions. Check for barrel exports hiding complex dependencies.

Java

Examine class hierarchies, interface implementations, and package dependencies. Look for "util" packages violating SRP.

Python

Check module organization, abstract base classes, and duck typing patterns. Verify protocol compliance.

Go

Analyze interface definitions (should be small), struct composition, and package dependencies.

PHP

Examine trait usage, interface implementations, and namespace organization.

Output Format

When reporting SOLID violations, structure findings as:

## SOLID Analysis Results

### Critical Violations

#### [File:Line] Principle Violated
- **Issue**: Description of the problem
- **Impact**: Why this matters
- **Suggestion**: How to fix it
- **Example**: Code snippet showing fix

### High Severity

...

Additional Resources

Reference Files

For detailed patterns and language-specific examples:

• references/violation-patterns.md - Comprehensive violation detection patterns
• references/refactoring-examples.md - Before/after code examples

Integration with Other Skills

Combine with:

• code-quality-metrics for complexity analysis
• refactoring-patterns for specific refactoring techniques