Cristian Sifuentes

Posted on Dec 23

C# Architecture Mastery — From OOP to SOLID to Clean Architecture (Part 2)

#dotnet #programming #csharp #cleanarchitecture

C# Architecture Mastery — From OOP to SOLID to Clean Architecture

Most developers learn OOP syntax early.

Few learn how OOP decisions scale into architecture.

In this Part 2, we move deliberately:

Objects → Principles → Architecture

This is the path senior .NET engineers follow—whether they name it or not.

1. OOP Is the Foundation, Not the Goal

OOP gives us:

Encapsulation
Abstraction
Inheritance
Polymorphism

But OOP alone does not guarantee good design.

Bad OOP still exists:

God classes
Tight coupling
Inheritance abuse
Anemic domain models

OOP is the toolset—not the blueprint.

2. Why SOLID Exists

SOLID exists because raw OOP breaks under scale.

Each principle answers a real pain point.

3. S — Single Responsibility Principle (SRP)

A class should have one reason to change.

What SRP really means

Not “one method.”

Not “small class.”

It means:

One axis of change
One business responsibility

// ❌ Violates SRP
class OrderService
{
    void CalculateTotal() { }
    void SaveToDatabase() { }
    void SendEmail() { }
}

// ✅ SRP-compliant
class OrderCalculator { }
class OrderRepository { }
class OrderNotifier { }

SRP improves:

Testability
Readability
Change isolation

4. O — Open / Closed Principle (OCP)

Open for extension, closed for modification.

The real goal

Avoid rewriting existing code when requirements change.

// ❌ Switch-based behavior
if (type == "A") { }
else if (type == "B") { }

// ✅ Polymorphic behavior
interface IDiscountStrategy
{
    decimal Apply(decimal price);
}

Add behavior by adding classes, not editing conditionals.

5. L — Liskov Substitution Principle (LSP)

Derived types must be substitutable for their base types.

If this breaks, polymorphism is lying.

// ❌ Violates LSP
class Bird
{
    virtual void Fly() { }
}

class Penguin : Bird
{
    override void Fly() => throw new Exception();
}

Fix the abstraction—not the override.

LSP enforces truthful inheritance.

6. I — Interface Segregation Principle (ISP)

Clients should not depend on methods they do not use.

// ❌ Fat interface
interface IMachine
{
    void Print();
    void Scan();
    void Fax();
}

// ✅ Segregated interfaces
interface IPrinter { void Print(); }
interface IScanner { void Scan(); }

ISP prevents:

Fake implementations
Empty methods
Accidental coupling

7. D — Dependency Inversion Principle (DIP)

Depend on abstractions, not concretions.

This is where architecture begins.

// ❌ Concrete dependency
class OrderService
{
    private readonly SqlOrderRepository _repo;
}

// ✅ Abstraction dependency
class OrderService
{
    private readonly IOrderRepository _repo;
}

DIP enables:

Dependency Injection
Test doubles
Modular systems

8. From SOLID to Clean Architecture

Clean Architecture applies SOLID at system scale.

Core rules:

Business logic does not depend on frameworks
UI and DB are details
Dependencies point inward

Layers:

Domain
Application
Infrastructure
UI

The domain is pure C#.

9. Clean Architecture Mental Model

Think in circles, not layers.

Inner circle = policies
Outer circle = details

Dependencies always point inward.

If your domain references Entity Framework:
👉 Your architecture is already broken.

10. A Senior-Level Checklist

Before shipping architecture:

Can I swap the database?
Can I test business logic without infrastructure?
Do dependencies flow inward?
Are abstractions meaningful?
Is behavior where it belongs?

If yes—you’re architecting, not just coding.

Final Thoughts

OOP teaches how to write objects.

SOLID teaches how to keep them sane.

Clean Architecture teaches how to scale systems without fear.

This is not academic theory.
This is how real-world .NET systems survive.

✍️ Written by Cristian Sifuentes — designing resilient .NET architectures and helping teams move from syntax to systems.

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