July 8, 2025 08:24 by
PeterSingle 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.
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