🧱 Mastering SOLID in .NET: From Messy Code to Professional Architecture

SOLID is a set of five object-oriented design principles that help developers write clean, maintainable, and scalable code. In this guide, we’ll explore each principle with:

✅ Concepts
⚠️ Common mistakes
🛠 Before & after code
💡 Improvement highlights

All examples are based on an e-commerce order system.

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1. S — Single Responsibility Principle (SRP)

"A class should have only one reason to change."

🧠 Concept:
A class should do only one thing and do it well. This keeps code focused and easier to maintain.

✅ Benefits:

• Modular and testable

• Easier to locate and fix bugs

⚠️ Problem:
OrderService mixes business logic, persistence, and notifications. Changes in one area could affect all others

❌ Not following SRP:

🔧 Solution (SRP applied):
We break the responsibilities into separate classes, increasing cohesion.

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2. O — Open/Closed Principle (OCP)

"Software entities should be open for extension, but closed for modification."

🧠 Concept:
You should be able to add behavior without changing existing code.

✅ Benefits:

• Adds features safely

• Reduces risk of regression

⚠️ Problem:
Every new discount type requires modifying a fragile if-based block.

❌ Not following OCP

🔧 Solution (OCP applied):
We use the strategy pattern so we can extend discount rules without touching core logic.

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3. L — Liskov Substitution Principle (LSP)

"Subtypes must be substitutable for their base types."

🧠 Concept:
Subclasses must honor the contracts of their base classes without breaking behavior.

✅ Benefits:

• Reliable inheritance

• Predictable usage of base types

⚠️ Problem:
SpecialOrder throws an exception when overridden method Cancel() is called. That’s a violation.

❌ Not following LSP

🔧 Solution (LSP applied):
We use abstract base class and require all child classes to define Cancel() safely

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4. I — Interface Segregation Principle (ISP)

"Clients should not be forced to depend on methods they do not use."

🧠 Concept:
Prefer smaller, purpose-driven interfaces over bloated ones.

✅ Benefits:

• Easier to implement

• More decoupled design

⚠️ Problem:
DigitalOrderProcessor doesn’t ship products — but still must implement ShipOrder().

❌ Not following ISP

🔧 Solution (ISP applied):
We break the interface into smaller parts. Each class implements only what it needs.

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5. D — Dependency Inversion Principle (DIP)

"Depend on abstractions, not concretions."

🧠 Concept:
High-level modules shouldn't depend on low-level classes — both should rely on abstractions.

✅ Benefits:

• Easier to test and mock

• More flexible architecture

⚠️ Problem:
OrderController depends directly on OrderRepository, making it hard to swap or test.

❌ Not following DIP

🔧 Solution (DIP applied):
We define an interface and inject the dependency — the controller is now loosely coupled.

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🚀 Final Thoughts

Applying SOLID principles in .NET is not about memorizing acronyms — it’s about thinking like a software designer. These principles help you build systems that are easier to evolve, test, and scale. Use them wisely, and your codebase will thank you.

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