SOLID vs Design Patterns

 

🔥 SOLID vs Design Patterns (Clear Mapping for .NET Core)

First: 1-Line Difference (Memorize This)

SOLID principles are rules to design good code.
Design Patterns are proven solutions that help follow those rules.

👉 SOLID = “What to follow”
👉 Design Patterns = “How to implement it”

---

🧠 Big Picture

SOLID Principle	Problem it Solves	Pattern(s) That Help
S – Single Responsibility	Too much logic in one class	Facade, Service, Decorator
O – Open/Closed	Code breaks when adding features	Strategy, Factory, Decorator
L – Liskov Substitution	Child classes break parent logic	Strategy, Template Method
I – Interface Segregation	Fat interfaces	Adapter, Decorator
D – Dependency Inversion	Tight coupling	Factory, Strategy, DI

Now let’s go one by one, with real .NET Core examples.

---

1️⃣ S – Single Responsibility Principle (SRP)

❌ Problem

One class does:

• Business logic

• Logging

• DB access

✅ Solution with Pattern

👉 Decorator / Facade

---

📌 Example: Logging without breaking SRP

public interface IOrderService
{
    void PlaceOrder();
}

public class OrderService : IOrderService
{
    public void PlaceOrder()
    {
        Console.WriteLine("Order placed");
    }
}

Add logging without changing OrderService:

public class OrderLoggingDecorator : IOrderService
{
    private readonly IOrderService _service;

    public OrderLoggingDecorator(IOrderService service)
    {
        _service = service;
    }

    public void PlaceOrder()
    {
        Console.WriteLine("Before order");
        _service.PlaceOrder();
        Console.WriteLine("After order");
    }
}

✔ OrderService → only business
✔ Logging → separate responsibility

🗣 Interview line

“Decorator helps maintain Single Responsibility by separating cross-cutting concerns.”

---

2️⃣ O – Open/Closed Principle (OCP)

❌ Problem

Every new feature = modify existing code.

✅ Solution with Pattern

👉 Strategy Pattern

---

📌 Example: Payment Logic

❌ Bad:

if(type == "Card") { }
else if(type == "UPI") { }

✅ Good (Strategy):

public interface IPaymentStrategy
{
    void Pay(decimal amount);
}

public class CardPayment : IPaymentStrategy
{
    public void Pay(decimal amount) { }
}

public class UpiPayment : IPaymentStrategy
{
    public void Pay(decimal amount) { }
}

✔ Add new payment → new class
✔ No existing code modified

🗣 Interview line

“Strategy pattern allows extending behavior without modifying existing code.”

---

3️⃣ L – Liskov Substitution Principle (LSP)

❌ Problem

Child class breaks parent behavior.

✅ Solution with Pattern

👉 Strategy Pattern

---

📌 Example

public interface IDiscountStrategy
{
    decimal Apply(decimal amount);
}

public class FlatDiscount : IDiscountStrategy
{
    public decimal Apply(decimal amount) => amount - 100;
}

public class PercentageDiscount : IDiscountStrategy
{
    public decimal Apply(decimal amount) => amount * 0.9m;
}

✔ Any strategy can replace another
✔ No unexpected behavior

🗣 Interview line

“Strategy pattern ensures Liskov Substitution because all implementations behave correctly.”

---

4️⃣ I – Interface Segregation Principle (ISP)

❌ Problem

Large interfaces force classes to implement unnecessary methods.

✅ Solution with Pattern

👉 Adapter Pattern

---

📌 Example

public interface IOldPrinter
{
    void Print();
    void Scan();
}

Modern printer only supports Print:

public class ModernPrinter
{
    public void Print() { }
}

Adapter:

public class PrinterAdapter : IOldPrinter
{
    private readonly ModernPrinter _printer = new();

    public void Print() => _printer.Print();
    public void Scan() => throw new NotSupportedException();
}

✔ Old interface preserved
✔ New class not forced to change

🗣 Interview line

“Adapter helps apply Interface Segregation when interfaces cannot be changed.”

---

5️⃣ D – Dependency Inversion Principle (DIP)

❌ Problem

High-level classes depend on concrete implementations.

✅ Solution with Pattern

👉 Factory + Strategy + Dependency Injection

---

📌 Example

public class NotificationService
{
    private readonly IMessageSender _sender;

    public NotificationService(IMessageSender sender)
    {
        _sender = sender;
    }

    public void Notify() => _sender.Send();
}

✔ Depends on abstraction
✔ Concrete implementation injected

builder.Services.AddScoped<IMessageSender, EmailSender>();

🗣 Interview line

“Dependency Inversion is implemented using interfaces, factories, and dependency injection.”

---

🧠 Final Mapping Table (Interview Gold)

SOLID	Pattern	Why
SRP	Decorator	Separate responsibilities
OCP	Strategy, Factory	Extend without modify
LSP	Strategy	Replace implementations safely
ISP	Adapter	Avoid fat interfaces
DIP	Factory, Strategy, DI	Depend on abstractions

---

🏆 Perfect Interview Answer (Memorize)

“SOLID principles guide good design, while design patterns help implement them. For example, Strategy and Factory help follow Open/Closed and Dependency Inversion, Decorator helps Single Responsibility, Adapter helps Interface Segregation, and Strategy ensures Liskov Substitution.”