✅ Single Responsibility Principle (SRP)
// One class = One job
Every class should have only one reason to change. This means no mixing database logic with business logic or API response formatting with validation rules.
💡 Real-World .NET Example
// Bad: Doing too much
public class UserManager
{
public void Register(User user) { /* logic */ }
public void SaveToDb(User user) { /* DB logic */ }
public void SendWelcomeEmail(User user) { /* Email logic */ }
}
// Good: SRP Applied
public class UserService
{
public void Register(UserDto dto) { /* business rules */ }
}
public class UserRepository
{
public void Save(User user) { /* DB only */ }
}
public class EmailSender
{
public void SendWelcomeEmail(User user) { /* Email only */ }
}
✅ Open/Closed Principle (OCP)
Open for extension, closed for modification
Your code should allow new behavior without changing existing code. This avoids regressions and makes it easier to add future features.
💡 Real-World Example
public interface IDiscountStrategy
{
decimal ApplyDiscount(decimal price);
}
public class HolidayDiscount : IDiscountStrategy
{
public decimal ApplyDiscount(decimal price) => price * 0.9m;
}
public class NoDiscount : IDiscountStrategy
{
public decimal ApplyDiscount(decimal price) => price;
}
By injecting IDiscountStrategy, you can add new discounts without changing your OrderService.
✅ Liskov Substitution Principle (LSP)
Subtypes must be substitutable for their base types
Any derived class should be replaceable for its parent without breaking functionality.
❌ Anti-Pattern
public class Rectangle
{
public virtual void SetWidth(int width) { }
}
public class Square : Rectangle
{
public override void SetWidth(int width)
{
throw new NotImplementedException(); // LSP Violation
}
}
Instead, design with interfaces or composition when inheritance breaks behavior.
✅ Interface Segregation Principle (ISP)
Keep interfaces small and focused
Clients shouldn't be forced to implement methods they don’t use. This avoids bloated and confusing code.
💡 Better Design
public interface ILoginService
{
void Login(string user, string password);
}
public interface IRegistrationService
{
void Register(UserDto dto);
}
Instead of forcing everything into a single IUserService, split interfaces by responsibility.
✅ Dependency Inversion Principle (DIP)
Depend on abstractions, not concrete classes
High-level modules (like OrderService) should depend on interfaces, not low-level classes like SqlOrderRepository.
💡 Real-World Example
public interface IEmailService
{
void SendEmail(string to, string message);
}
public class EmailService : IEmailService
{
public void SendEmail(string to, string message)
{
// SMTP logic
}
}
public class NotificationManager
{
private readonly IEmailService _emailService;
public NotificationManager(IEmailService emailService)
{
_emailService = emailService;
}
public void Notify(string msg)
{
_emailService.SendEmail("[email protected]", msg);
}
}
In Program.cs
services.AddScoped<IEmailService, EmailService>();
🚀 How to Apply SOLID Principles Efficiently While Working?
- Start with interfaces, then build implementations.
- Don’t mix responsibilities: Separate service, repository, controller.
- Write unit tests: If a class is hard to test, it’s probably violating SOLID.
- Utilize design patterns:such as Strategy, Factory, and Mediator, to simplify SOLID principles.
- Think like Lego: Compose functionality from small, testable bricks.
🌟 Final Thoughts
Mastering SOLID is like upgrading your dev brain. You’ll not only write clean and testable code but also scale faster, debug smarter, and survive enterprise-level chaos like a pro.
Want feedback on how SOLID your current codebase is? Drop a sample, and let's refactor it together!
Happy coding, and stay SOLID ✌️