Claude Skills Guide

Claude Code for Dependency Inversion Refactoring Guide

Dependency Inversion is one of the most transformative principles in software design, yet applying it to existing codebases can feel overwhelming. Fortunately, Claude Code transforms this refactoring from a manual, error-prone process into an assisted, systematic approach. This guide shows you how to use Claude Code effectively for dependency inversion refactoring.

Dependency Inversion vs. Dependency Injection: Know the Difference

These two terms are closely related but address different concerns.

Dependency Inversion Principle (DIP) is a design principle—one of the five SOLID principles. It states that high-level modules should not depend on low-level modules; both should depend on abstractions. This is an architectural rule about how you structure your codebase.

Dependency Injection (DI) is a technique for satisfying dependencies from outside a class rather than having the class instantiate them itself. DI is a common implementation mechanism for achieving DIP, but the two are not the same thing.

This guide is about using Claude Code to apply DIP when refactoring application-level code—TypeScript services, repositories, and business logic. If you are instead looking for how to apply DI patterns within Claude skills themselves (skill composition, parameterized skill invocation, tool abstraction layers), see the dedicated guide: Claude Code Dependency Injection Refactoring.

Understanding Dependency Inversion

Before diving into the refactoring process, let’s clarify what Dependency Inversion means:

In practical terms, this means your business logic shouldn’t directly instantiate or call concrete implementations. Instead, both should depend on interfaces or abstract classes.

Initial Assessment with Claude Code

Start by having Claude analyze your codebase to identify dependency violations:

claude "Find direct database calls, file system operations, or HTTP client instantiations 
in the business logic layer. List each file and the specific lines where concrete 
dependencies are injected or instantiated directly."

Claude will scan your codebase and provide a concrete list of areas needing attention. This gives you a refactoring roadmap.

The Refactoring Workflow

Step 1: Identify the Dependency

Ask Claude to examine a specific class with direct dependencies:

claude "Analyze UserService in src/services/UserService.ts. Identify all 
direct dependencies on concrete classes (database, HTTP clients, file systems). 
Show the current implementation and explain each dependency violation."

Claude will show you code like this problematic example:

// Before: Direct dependency on concrete class
class UserService {
  private database = new PostgreSQLDatabase(); // ❌ Violation
  
  async getUser(id: string) {
    return this.database.query('SELECT * FROM users WHERE id = ?', id);
  }
}

Step 2: Define the Abstraction

Ask Claude to create the interface:

claude "Create an interface for the database operations that UserService needs.
The interface should define methods for querying by ID, inserting, updating, 
and deleting users. Place it in src/interfaces/."

Claude will generate:

// After: Depend on abstraction
interface IUserRepository {
  findById(id: string): Promise<User | null>;
  save(user: User): Promise<void>;
  delete(id: string): Promise<void>;
}

Step 3: Refactor the Service

Have Claude refactor the service to depend on the interface:

claude "Refactor UserService to depend on IUserRepository instead of the 
concrete PostgreSQLDatabase. Use constructor injection. Make sure to keep 
the same public API."

The result:

class UserService {
  constructor(private userRepository: IUserRepository) {}
  
  async getUser(id: string) {
    return this.userRepository.findById(id);
  }
}

Step 4: Implement the Concrete Dependency

Ask Claude to create the implementation:

claude "Create PostgreSQLUserRepository that implements IUserRepository 
in src/repositories/. It should wrap the existing PostgreSQLDatabase logic."

Handling Constructor Injection

One common challenge is managing constructor injection, especially with many dependencies. Ask Claude:

claude "Refactor OrderProcessor to use constructor injection for its 
dependencies: EmailService, PaymentGateway, and InventoryService. Use 
TypeScript private readonly properties. Show the before and after."

For dependency injection containers, ask:

claude "Create a simple dependency injection container in TypeScript 
that can register and resolve services by their interface. Include 
singleton and transient registration options."

Testing Benefits

One of the greatest advantages of dependency inversion is testability. Ask Claude to demonstrate:

claude "Create a mock implementation of IUserRepository for testing 
UserService. Show how to use it in a Jest test to verify getUser calls 
the repository and handles null returns correctly."

You’ll get:

// Mock repository for testing
class MockUserRepository implements IUserRepository {
  private users: Map<string, User> = new Map();
  
  async findById(id: string): Promise<User | null> {
    return this.users.get(id) || null;
  }
  
  async save(user: User): Promise<void> {
    this.users.set(user.id, user);
  }
  
  async delete(id: string): Promise<void> {
    this.users.delete(id);
  }
}

// Test
describe('UserService', () => {
  it('returns user when found', async () => {
    const mockRepo = new MockUserRepository();
    mockRepo.save({ id: '1', name: 'John' });
    const service = new UserService(mockRepo);
    
    const user = await service.getUser('1');
    expect(user?.name).toBe('John');
  });
});

Common Pitfalls to Avoid

1. Interface Pollution

Don’t create interfaces for every single class. Ask Claude to help you identify when an interface is truly needed:

claude "Review these classes and suggest which need interfaces vs which 
can be used directly. Consider: EmailService, StringUtils, DateHelper, 
PaymentProcessor."

2. Over-Abstraction

If you find yourself creating interfaces with single implementations and no testing need, you’ve over-abstracted. Ask:

claude "Is this interface necessary? Review ILoggingService with single 
implementation ConsoleLogger. Should this be an interface or a concrete class?"

3. Leaking Implementations

Sometimes concrete classes leak into abstractions. Ask Claude to check:

claude "Review INotificationService interface. Are there any method 
signatures that expose implementation details? For example, using 
SendGridResponse or SMTPConfig in the interface would be a violation."

Automation Strategies

For large codebases, refactor systematically:

claude "Find all classes in src/services/ that directly instantiate 
database connections. List them and categorize by complexity (single 
vs multiple dependencies)."

Then refactor one category at a time:

claude "Starting with AuthenticationService, refactor all services in 
the authentication module to use dependency injection. Create interfaces 
in src/interfaces/, update the services, and ensure the module still works."

Conclusion

Claude Code transforms dependency inversion refactoring from a daunting manual task into a guided, systematic process. By using Claude’s ability to analyze code, generate interfaces, and refactor implementations, you can:

Start with the most critical business logic classes, apply the workflow systematically, and enjoy the improved flexibility and testability that dependency inversion brings.

Remember: the goal isn’t abstraction for its own sake—it’s about decoupling your business logic from implementation details so changes don’t cascade through your system.

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