A Complete Beginner-Friendly Guide to Understanding SOLID Principles in C# with Practical Real-World Examples

SOLID Principles in C#: Writing Clean, Scalable, and Maintainable Code

Table of Contents

• What Are SOLID Principles?
• Single Responsibility Principle (SRP)
• Open/Closed Principle (OCP)
• Liskov Substitution Principle (LSP)
• Interface Segregation Principle (ISP)
• Dependency Inversion Principle (DIP)
• Conclusion

⭐ What Are SOLID Principles?

SOLID principles are a set of five object-oriented design principles created to help developers write cleaner, more flexible, and maintainable code. They reduce bugs, improve readability, and make applications easier to scale.

• S – Single Responsibility Principle
• O – Open/Closed Principle
• L – Liskov Substitution Principle
• I – Interface Segregation Principle
• D – Dependency Inversion Principle

Tip: SOLID is widely used in enterprise-level .NET applications and interviews!

1️⃣ Single Responsibility Principle (SRP)

A class should have only one reason to change. That means it must do exactly *one* job.

❌ Bad Example

public class Invoice
{
    public void CalculateTotal() { }
    public void SaveToDatabase() { }
    public void GeneratePDF() { }
}

✔ Good Example

public class InvoiceCalculator { public void Calculate() { } }
public class InvoiceRepository { public void Save() { } }
public class InvoicePrinter { public void Print() { } }

2️⃣ Open/Closed Principle (OCP)

Classes should be open for extension but closed for modification. You shouldn't modify existing code to add new behavior—extend it!

✔ Example Using Polymorphism

public abstract class Discount
{
    public abstract double Apply(double price);
}

public class NewCustomerDiscount : Discount
{
    public override double Apply(double price) => price * 0.90;
}

public class VIPDiscount : Discount
{
    public override double Apply(double price) => price * 0.80;
}

3️⃣ Liskov Substitution Principle (LSP)

Subclasses should replace their base classes without breaking functionality.

❌ Violation Example

public class Bird { public virtual void Fly() { } }

public class Ostrich : Bird
{
    public override void Fly() { throw new Exception("Cannot fly!"); }
}

✔ Correct Approach

public abstract class Bird { }
public abstract class FlyingBird : Bird { public abstract void Fly(); }
public class Sparrow : FlyingBird { public override void Fly() { } }
public class Ostrich : Bird { }

4️⃣ Interface Segregation Principle (ISP)

Clients should not depend on interfaces they do not use.

❌ Bad Interface

public interface IWorker
{
    void Work();
    void Eat();
}

✔ Better Approach

public interface IWork { void Work(); }
public interface IEat { void Eat(); }

5️⃣ Dependency Inversion Principle (DIP)

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

❌ Bad Example

public class Notification
{
    EmailService _service = new EmailService();
    public void Send() { _service.SendEmail(); }
}

✔ Good Example

public interface IMessageService
{
    void Send();
}

public class EmailService : IMessageService
{
    public void Send() { }
}

public class Notification
{
    private readonly IMessageService _service;
    public Notification(IMessageService service) { _service = service; }
    public void Send() => _service.Send();
}

Conclusion

The SOLID principles help C# developers build flexible, scalable, and maintainable software systems. By following these principles, you can reduce code smells, avoid unnecessary dependencies, and write clean object-oriented code that is easier to test and extend.

Start applying SOLID principles in your next .NET project and experience the difference in quality and maintainability!

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