SOLID Principles: Writing Better Object-Oriented Code

What are SOLID Principles?

SOLID is an acronym for five design principles introduced by Robert C. Martin (Uncle Bob) that make object-oriented designs more understandable, flexible, and maintainable.

S — Single Responsibility Principle

A class should have only one reason to change. It should do one thing and do it well.

// Bad: One class doing too much
class UserService {
    public void CreateUser() { }
    public void SendWelcomeEmail() { }
    public void LogActivity() { }
}

// Good: Separated responsibilities
class UserService { public void CreateUser() { } }
class EmailService { public void SendWelcomeEmail() { } }
class AuditLogger { public void LogActivity() { } }

O — Open/Closed Principle

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

abstract class Discount {
    public abstract decimal Apply(decimal price);
}
class SeasonalDiscount : Discount {
    public override decimal Apply(decimal price) => price * 0.9m;
}
// Add new discount types without modifying existing code

L — Liskov Substitution Principle

Objects of a subclass should be replaceable by objects of their superclass without altering correctness.

I — Interface Segregation Principle

No client should be forced to depend on interfaces it does not use. Prefer many small, specific interfaces over one large general-purpose one.

// Bad
interface IWorker { void Work(); void Eat(); void Sleep(); }

// Good
interface IWorkable { void Work(); }
interface IFeedable { void Eat(); }

D — Dependency Inversion Principle

High-level modules should not depend on low-level modules. Both should depend on abstractions.

// Depend on abstraction, not concrete class
class OrderService {
    private readonly IPaymentGateway _gateway;
    public OrderService(IPaymentGateway gateway) { _gateway = gateway; }
    public void ProcessOrder() => _gateway.Charge();
}

Conclusion

Applying SOLID principles leads to code that is easier to test, extend, and maintain. They are the backbone of clean architecture in any OOP language.