Cristian Sifuentes

Posted on Dec 22

1 - Classes in C#: From First Principles to Architectural Mastery

#csharp #dotnet #oop #architecture

Classes in C#: From First Principles to Architectural Mastery

An artist–scientist guide to object-oriented design in modern C#

Good C# design is not about knowing syntax — it’s about choosing the right class type intentionally.

Classes are not just language constructs. They are decisions about memory, identity, evolution, and time.

This article blends computer science fundamentals, real-world architecture, and mental models rarely explained explicitly.

If you’ve ever wondered why certain designs feel robust while others decay — this is for you.

Why Classes Exist at All (The Memory Perspective)

Before OOP, before patterns, before frameworks — there is RAM.

A class is a blueprint for memory.

An object is a configured block of memory.
A class defines:
- how big that block is
- what data lives inside it
- what operations are allowed on it

The bookshelf model (expanded)

RAM → the bookshelf
Classes → book formats
Objects → individual books
Fields → pages
Methods → how the book can be read or modified

Different class types exist because not all memory needs identity, mutability, or inheritance.

The 8 Types of Classes — A Mental Model

Each class type answers one architectural question:

“How should this behavior or data exist in my system?”

1️⃣ Concrete Class (Normal Class)

Question it answers

👉 Do I need an object with state and behavior?

public class Employee
{
    public string Name { get; set; }

    public void Display()
    {
        Console.WriteLine($"Employee Name: {Name}");
    }
}

Key traits

Instantiable
Stateful
Inheritable
Can implement interfaces

Deep insight (rarely discussed)

Concrete classes represent identity over time.

They are ideal when mutation is meaningful.

Real-world usage

Domain entities (Order, User)
Services (EmailService)
Controllers, processors, handlers

✅ Default choice — until constraints are needed.

2️⃣ Static Class

Question it answers

👉 Does this logic need identity or memory ownership?

public static class MathHelper
{
    public static int Add(int a, int b) => a + b;
}

Key traits

No instances
No inheritance
Stateless by design

Deep insight

Static classes are pure behavior, not actors.

They are closer to mathematical functions than objects.

Real-world usage

Utility functions
Extension methods
Deterministic transformations

⚠️ Smell check

If you want state → you want an object, not a static class.

3️⃣ Abstract Class

Question it answers

👉 What rules must all derived types obey?

public abstract class Shape
{
    public abstract void Draw();
}

public class Circle : Shape
{
    public override void Draw() => Console.WriteLine("Drawing Circle");
}

Key traits

Cannot be instantiated
Can hold state
Can enforce workflow

Deep insight

Abstract classes encode temporal invariants —

they control how behavior unfolds over time.

🧠 Rule of thumb

Abstract class = identity + rules + shared behavior

4️⃣ Sealed Class

Question it answers

👉 Should this type ever evolve via inheritance?

public sealed class Logger
{
    public void Log(string message) => Console.WriteLine(message);
}

Key traits

No inheritance allowed
Predictable behavior
Easier to reason about

Deep insight

sealed is about epistemic safety:

you know exactly what this type can do — forever.

5️⃣ Partial Class

Question it answers

👉 How do humans and machines co-author the same type?

// File1.cs
public partial class Student
{
    public string Name { get; set; }
}

// File2.cs
public partial class Student
{
    public void Display() => Console.WriteLine(Name);
}

Key traits

One type, many files
Compiler stitches them together

Deep insight

Partial classes are sociotechnical tools —

they exist to allow humans and generators to collaborate.

6️⃣ Nested Class

Question it answers

👉 Should this type exist outside its parent?

public class Outer
{
    public class Inner
    {
        public void Show() => Console.WriteLine("Inner Class");
    }
}

Key traits

Scoped visibility
Access to private members of outer class

Deep insight

Nested classes encode ontological dependency:

This thing cannot exist meaningfully on its own.

7️⃣ Generic Class

Question it answers

👉 Can this logic be universal without losing safety?

public class DataStore<T>
{
    public T Data { get; set; }
}

Key traits

Compile-time polymorphism
Zero runtime casting
High reuse

Deep insight

Generics are parametric polymorphism —

a mathematical concept brought into programming.

8️⃣ Anonymous Class

Question it answers

👉 Do I need a temporary data shape right now?

var person = new { Name = "John", Age = 25 };
Console.WriteLine(person.Name);

Key traits

No name
Immutable
Method-scoped

Deep insight

Anonymous types are ephemeral structures —

they exist to think, not to build.

How These 8 Types Fit Together

Purpose	Best Choice
Domain behavior	Concrete class
Utilities	Static class
Shared rules	Abstract class
Lock behavior	Sealed class
Tool-generated code	Partial class
Scoped helpers	Nested class
Reusable logic	Generic class
Temporary data	Anonymous class