SOLID Principles

Object-Oriented Programming

SOLID principles introduced and popularized by Robert C. Martin (“Uncle Bob”) in the early 2000s, with the acronym coined by Michael Feathers

SOLID principles create maintainable, scalable, and flexible software systems that are easier to understand, modify, and extend over time.

Historical context: While Martin popularized these under the SOLID acronym, the individual principles have earlier origins:

Single Responsibility: Tom DeMarco & Meilir Page-Jones (cohesion concepts, 1970s-80s)
Open-Closed: Bertrand Meyer (1988, “Object-Oriented Software Construction”)
Liskov Substitution: Barbara Liskov (1987, data abstraction keynote)
Interface Segregation: Robert C. Martin (1990s)
Dependency Inversion: Robert C. Martin (1990s)

S - Single Responsibility Principle (SRP)

Definition: A class should have only one reason to change, meaning it should have only one job or responsibility.

Intent: Each class should focus on a single concern, making it easier to understand, test, and maintain.

Benefits:

Easier to test and maintain
Reduces coupling between components
Simplifies debugging and troubleshooting
Clearer code organization

Example Violation:

// BAD: Multiple responsibilities
public class Employee
{
    public string Name { get; set; }
    public decimal Salary { get; set; }

    // Responsibility 1: Salary calculation
    public decimal CalculateSalary()
    {
        return Salary * 1.1m; // 10% bonus
    }

    // Responsibility 2: Report generation
    public string GeneratePayrollReport()
    {
        return $"Employee: {Name}, Salary: ${CalculateSalary():F2}";
    }

    // Responsibility 3: Data persistence
    public void SaveToDatabase()
    {
        // Database logic
        var connection = new SqlConnection("...");
        // Save employee data
    }

    // Responsibility 4: Email notifications
    public void SendPayStub()
    {
        // Email sending logic
        var emailService = new SmtpClient();
        // Send email
    }
}

Better Approach:

// GOOD: Single responsibilities
public class Employee
{
    public string Name { get; set; }
    public decimal BaseSalary { get; set; }
    public string Email { get; set; }
}

public class SalaryCalculator
{
    public decimal Calculate(Employee employee)
    {
        return employee.BaseSalary * 1.1m;
    }
}

public class PayrollReporter
{
    private readonly SalaryCalculator calculator;

    public PayrollReporter(SalaryCalculator calculator)
    {
        this.calculator = calculator;
    }

    public string GenerateReport(Employee employee)
    {
        var salary = calculator.Calculate(employee);
        return $"Employee: {employee.Name}, Salary: ${salary:F2}";
    }
}

public class EmployeeRepository
{
    private readonly string connectionString;

    public EmployeeRepository(string connectionString)
    {
        this.connectionString = connectionString;
    }

    public void Save(Employee employee)
    {
        using var connection = new SqlConnection(connectionString);
        // Save employee data
    }
}

public class PayStubNotifier
{
    private readonly IEmailService emailService;

    public PayStubNotifier(IEmailService emailService)
    {
        this.emailService = emailService;
    }

    public async Task SendPayStub(Employee employee, string payStubContent)
    {
        await emailService.SendAsync(employee.Email, "Pay Stub", payStubContent);
    }
}

When to Apply: Ask “What is the single reason this class would change?” If there are multiple answers, refactor.

O - Open-Closed Principle (OCP)

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

Intent: Design classes so new functionality can be added without changing existing code, reducing risk of bugs.

Benefits:

Add new features without changing existing code
Reduces risk of introducing bugs in working code
Promotes code reusability
Supports plugin architectures

Example Violation:

// BAD: Must modify class to add new shapes
public class AreaCalculator
{
    public double CalculateArea(object shape)
    {
        if (shape is Circle circle)
        {
            return Math.PI * circle.Radius * circle.Radius;
        }
        else if (shape is Rectangle rectangle)
        {
            return rectangle.Width * rectangle.Height;
        }
        // Adding Triangle requires modifying this method
        else if (shape is Triangle triangle)
        {
            return 0.5 * triangle.Base * triangle.Height;
        }

        throw new ArgumentException("Unknown shape");
    }
}

Better Approach:

// GOOD: Open for extension, closed for modification
public interface IShape
{
    double CalculateArea();
}

public class Circle : IShape
{
    public double Radius { get; set; }
    public double CalculateArea() => Math.PI * Radius * Radius;
}

public class Rectangle : IShape
{
    public double Width { get; set; }
    public double Height { get; set; }
    public double CalculateArea() => Width * Height;
}

public class Triangle : IShape
{
    public double Base { get; set; }
    public double Height { get; set; }
    public double CalculateArea() => 0.5 * Base * Height;
}

// No modification needed when adding new shapes
public class AreaCalculator
{
    public double CalculateTotalArea(IEnumerable<IShape> shapes)
    {
        return shapes.Sum(s => s.CalculateArea());
    }
}

// New shape can be added without modifying existing code
public class Hexagon : IShape
{
    public double SideLength { get; set; }
    public double CalculateArea() => (3 * Math.Sqrt(3) / 2) * Math.Pow(SideLength, 2);
}

When to Apply: Use abstractions (interfaces/base classes) when you anticipate multiple implementations or variations.

L - Liskov Substitution Principle (LSP)

Introduced by Barbara Liskov in her 1987 keynote “Data Abstraction and Hierarchy” at OOPSLA. Formalized with Jeannette Wing in 1994.

Definition: Objects of a superclass should be replaceable with objects of a subclass without altering the correctness of the program.

Formal definition (Liskov & Wing): If S is a subtype of T, then objects of type T may be replaced with objects of type S without breaking the program.

Intent: Subtypes must be substitutable for their base types without breaking functionality.

Benefits:

Ensures polymorphism works correctly
Maintains contract integrity
Enables reliable inheritance hierarchies
Prevents unexpected behavior

Key Rule: Subclasses must:

Strengthen postconditions (can return more specific types)
Weaken preconditions (can accept more general parameters)
Preserve invariants (maintain class constraints)
Not throw new exceptions on base methods

Example Violation:

// BAD: Violates LSP
public class Bird
{
    public virtual void Fly()
    {
        Console.WriteLine("Flying high!");
    }
}

public class Sparrow : Bird
{
    public override void Fly()
    {
        Console.WriteLine("Sparrow flies!");
    }
}

public class Penguin : Bird
{
    public override void Fly()
    {
        // Penguins can't fly - violates LSP!
        throw new NotSupportedException("Penguins can't fly!");
    }
}

// This breaks when using Penguin
public void MakeBirdFly(Bird bird)
{
    bird.Fly(); // Will throw exception for Penguin!
}

Better Approach:

// GOOD: Follows LSP
public abstract class Bird
{
    public abstract void Move();
}

public interface IFlyable
{
    void Fly();
}

public class Sparrow : Bird, IFlyable
{
    public override void Move()
    {
        Fly();
    }

    public void Fly()
    {
        Console.WriteLine("Sparrow flies!");
    }
}

public class Penguin : Bird
{
    public override void Move()
    {
        Swim();
    }

    public void Swim()
    {
        Console.WriteLine("Penguin swims!");
    }
}

// Works correctly with all birds
public void MakeBirdMove(Bird bird)
{
    bird.Move(); // Works for all bird types
}

// Only works with flyable birds
public void MakeFlyableFly(IFlyable flyable)
{
    flyable.Fly(); // Only accepts birds that can fly
}

When to Apply: Ensure derived classes can truly replace base classes without breaking client code.

I - Interface Segregation Principle (ISP)

Definition: Clients should not be forced to depend on interfaces they don’t use.

Intent: Create small, focused interfaces rather than large, monolithic ones.

Benefits:

Reduces coupling between components
Avoids unnecessary dependencies
Enables more targeted implementations
Improves code clarity

Example Violation:

// BAD: Fat interface forces implementations to support all methods
public interface IWorker
{
    void Work();
    void Eat();
    void Sleep();
    void GetPaid();
}

public class HumanWorker : IWorker
{
    public void Work() { Console.WriteLine("Working..."); }
    public void Eat() { Console.WriteLine("Eating lunch..."); }
    public void Sleep() { Console.WriteLine("Sleeping..."); }
    public void GetPaid() { Console.WriteLine("Getting paid!"); }
}

public class RobotWorker : IWorker
{
    public void Work() { Console.WriteLine("Working..."); }

    // Forced to implement methods that don't make sense for robots
    public void Eat() { throw new NotSupportedException(); }
    public void Sleep() { throw new NotSupportedException(); }
    public void GetPaid() { throw new NotSupportedException(); }
}

Better Approach:

// GOOD: Segregated interfaces
public interface IWorkable
{
    void Work();
}

public interface IEatable
{
    void Eat();
}

public interface ISleepable
{
    void Sleep();
}

public interface IPayable
{
    void GetPaid();
}

public class HumanWorker : IWorkable, IEatable, ISleepable, IPayable
{
    public void Work() { Console.WriteLine("Working..."); }
    public void Eat() { Console.WriteLine("Eating lunch..."); }
    public void Sleep() { Console.WriteLine("Sleeping..."); }
    public void GetPaid() { Console.WriteLine("Getting paid!"); }
}

public class RobotWorker : IWorkable
{
    public void Work() { Console.WriteLine("Working tirelessly..."); }
    // Only implements what makes sense
}

// Clients depend only on what they need
public class WorkManager
{
    public void ManageWork(IWorkable worker)
    {
        worker.Work(); // Only requires IWorkable
    }
}

public class PayrollManager
{
    public void ProcessPayroll(IPayable employee)
    {
        employee.GetPaid(); // Only requires IPayable
    }
}

When to Apply: When interfaces become large, split them into smaller, focused interfaces based on client needs.

D - Dependency Inversion Principle (DIP)

Definition: 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.

Intent: Decouple high-level business logic from low-level implementation details.

Benefits:

Improves testability through dependency injection
Reduces coupling between layers
Enables flexible architecture
Supports multiple implementations

Example Violation:

// BAD: High-level class depends on low-level implementation
public class EmailNotification
{
    public void Send(string to, string message)
    {
        // Concrete SMTP implementation
        var smtpClient = new SmtpClient("smtp.gmail.com");
        smtpClient.Send(to, message);
    }
}

public class OrderService
{
    // Tightly coupled to EmailNotification
    private readonly EmailNotification emailNotification = new EmailNotification();

    public void PlaceOrder(Order order)
    {
        // Process order...
        emailNotification.Send(order.CustomerEmail, "Order placed!");
    }
}

Better Approach:

// GOOD: Both depend on abstraction
public interface INotificationService
{
    Task SendAsync(string recipient, string message);
}

// Low-level implementation
public class EmailNotificationService : INotificationService
{
    private readonly string smtpServer;

    public EmailNotificationService(string smtpServer)
    {
        this.smtpServer = smtpServer;
    }

    public async Task SendAsync(string recipient, string message)
    {
        var smtpClient = new SmtpClient(smtpServer);
        await smtpClient.SendMailAsync(recipient, message);
    }
}

// Alternative implementation
public class SmsNotificationService : INotificationService
{
    private readonly string apiKey;

    public SmsNotificationService(string apiKey)
    {
        this.apiKey = apiKey;
    }

    public async Task SendAsync(string recipient, string message)
    {
        var smsClient = new TwilioClient(apiKey);
        await smsClient.SendSmsAsync(recipient, message);
    }
}

// High-level module depends on abstraction
public class OrderService
{
    private readonly INotificationService notificationService;

    // Dependency injected through constructor
    public OrderService(INotificationService notificationService)
    {
        this.notificationService = notificationService;
    }

    public async Task PlaceOrder(Order order)
    {
        // Process order...
        await notificationService.SendAsync(order.CustomerEmail, "Order placed!");
    }
}

// Configuration
var emailService = new EmailNotificationService("smtp.gmail.com");
var orderService = new OrderService(emailService);

// Easy to swap implementations
var smsService = new SmsNotificationService("twilio-api-key");
var orderServiceWithSms = new OrderService(smsService);

When to Apply: Always depend on abstractions when working across architectural boundaries (layers, services, components).

Quick Reference

SOLID Principles Comparison

Principle	Focus	Key Question
SRP	Class cohesion	“Does this class have one reason to change?”
OCP	Extension mechanism	“Can I add features without modifying existing code?”
LSP	Inheritance contracts	“Can I substitute derived classes for base classes?”
ISP	Interface granularity	“Does this interface force unnecessary dependencies?”
DIP	Dependency direction	“Am I depending on abstractions or implementations?”

Common Violations and Fixes

Violation	Sign	Fix
SRP	“And” in class names, many imports	Extract classes for each responsibility
OCP	Long if/switch statements on type	Use polymorphism via interfaces
LSP	Throwing exceptions in overrides	Redesign hierarchy or use composition
ISP	NotImplementedException in interface methods	Split into smaller interfaces
DIP	New keyword everywhere, hard to test	Use dependency injection

SOLID in Modern Development

ASP.NET Core:

SRP: Controllers handle HTTP, services handle business logic
OCP: Middleware pipeline extends via new middleware
LSP: Custom implementations replace defaults seamlessly
ISP: Small interfaces like IHostedService, IHealthCheck
DIP: Built-in dependency injection container

Testing Benefits:

SRP: Test one thing at a time
OCP: Test new features independently
LSP: Test base types, trust derived types
ISP: Mock only what’s needed
DIP: Inject test doubles easily