Design Patterns

Dependency Inversion Principle: The 'D' in SOLID

nemostorm•January 11, 2026•10 min read

Dependency Inversion Principle: The 'D' in SOLID

What is the Dependency Inversion Principle?

The Dependency Inversion Principle (DIP) is the fifth and final principle in SOLID. It states:

"High-level modules should not depend on low-level modules. Both should depend on abstractions. Abstractions should not depend on details. Details should depend on abstractions."

In simpler terms: Depend on interfaces or abstract classes, not concrete implementations.

The Problem Without DIP

Consider an e-commerce application that needs to send notifications:

csharp

public class EmailService
{
    public void SendEmail(string to, string subject, string body)
    {
        Console.WriteLine($"Sending email to {to}");
        Console.WriteLine($"Subject: {subject}");
        Console.WriteLine($"Body: {body}");
    }
}

public class OrderProcessor
{
    private readonly EmailService _emailService;

    public OrderProcessor()
    {
        _emailService = new EmailService();
    }

    public void ProcessOrder(Order order)
    {
        // Process order logic
        Console.WriteLine($"Processing order {order.Id}");
        
        // Send confirmation
        _emailService.SendEmail(
            order.CustomerEmail,
            "Order Confirmation",
            $"Your order {order.Id} has been processed"
        );
    }
}

The Solution: Dependency Inversion

The interactive code editor above demonstrates the proper implementation of the Dependency Inversion Principle:

1. Define Abstractions: Create interfaces that define contracts without implementation details 2. Implement Concrete Classes: Create classes that implement these interfaces 3. Depend on Abstractions: High-level modules depend on interfaces, not concrete implementations 4. Use Dependency Injection: Inject implementations at runtime through constructors or properties

Benefits of DIP

Loose Coupling: Modules are independent and can be changed without affecting others
Testability: Easy to mock dependencies for unit testing
Flexibility: Can swap implementations without changing high-level code
Maintainability: Changes to low-level modules don't break high-level modules

Conclusion

The Dependency Inversion Principle is fundamental to building maintainable, testable, and flexible software. By depending on abstractions rather than concrete implementations, you create loosely coupled systems that are easier to modify, test, and extend.

Key takeaways:

Depend on interfaces/abstractions, not concrete classes
Use dependency injection to provide implementations
Create abstractions only when they add value
Focus on testability and flexibility

When combined with the other SOLID principles, DIP helps you build robust, professional-grade applications that stand the test of time.