SOLID Principles

Overview

SOLID is a set of five design principles that improve maintainability, testability, and extensibility of object-oriented systems.

The Five Principles

• Single Responsibility Principle (SRP)
• Open/Closed Principle (OCP)
• Liskov Substitution Principle (LSP)
• Interface Segregation Principle (ISP)
• Dependency Inversion Principle (DIP)

flowchart LR
    A[SRP] --> F[Maintainable Design]
    B[OCP] --> F
    C[LSP] --> F
    D[ISP] --> F
    E[DIP] --> F

Practical Example

A payment flow often violates SRP when one class handles validation, charging, and notifications.

public class PaymentProcessor
{
    public void Process(Order order)
    {
        Validate(order);
        Charge(order);
        SendConfirmation(order);
    }

Refactor to separate responsibilities:

public class PaymentValidator
{
    public void Validate(Order order) { }
}

public class PaymentGateway
{
    public void Charge(Order order) { }
}

public class ReceiptNotifier
{
    public void SendConfirmation(Order order) { }
}

Production Guidance

• Apply DIP for external systems through interfaces
• Keep interfaces focused on client needs
• Favor composition over deep inheritance trees

Common Pitfalls

• Overengineering small modules with unnecessary abstractions
• Creating interfaces before concrete use cases exist
• Treating SOLID as strict rules instead of design guidance

Key Takeaways

• SOLID helps code evolve safely under changing requirements.
• SRP and DIP typically provide the fastest practical gains.
• Principles should support delivery speed, not reduce it.