C# OOP Mastery — From Quiz Answers to Production‑Grade Mental Models

Source: Dev.to

Most C# interviews won’t ask you to build a whole enterprise system.
Instead, they quietly test whether you actually understand the mental model behind OOP fundamentals:

• What’s the difference between defining a class and creating an object?
• Why do methods represent behavior?
• Can C# inherit from multiple classes?
• Why do we use constructors (and when do we prefer object initializers)?
• What do access modifiers actually control?
• Why are record types “special” in modern C#?

In this post we’ll take quiz‑style questions and turn them into production patterns you can reuse in:

• code reviews
• architecture discussions
• technical interviews
• real .NET services

TL;DR — What you’ll learn

• The type vs. instance mental model (class vs. new)
• Why methods are behavior, not “just functions”
• The actual rule: single‑class inheritance, multiple interfaces
• Constructors, object initializers, and invariants
• Encapsulation with properties: get; set; (and how seniors restrict setters)
• Access modifiers: not “what exists”, but who can touch it
• protected reality (and when it becomes a design smell)
• Why record exists: value equality, immutability, and data modeling

1) “Which keyword creates an object?” — Definition vs. Instantiation

Quiz version

“What is the keyword or directive to create an object in C#?”

✅ Answer: new

Senior mental model

• class defines a type (a blueprint)
• new creates an instance (a runtime object on the managed heap, initialized by a constructor)

public class Person
{
    public string Name { get; }
    public Person(string name) => Name = name;
}

var p = new Person("Carlos"); // ✅ creates an object (instance)

Interview soundbite

“class defines the blueprint; new builds a real instance and runs the constructor.”

2) “What do methods define inside a class?” — Behavior

Quiz version

“What do methods define within a class?”

✅ Answer: Behavior

Why this matters in production

• Properties/fields represent state.
• Methods represent behavior—the rules for changing that state.

public sealed class BankAccount
{
    public decimal Balance { get; private set; }

    public void Deposit(decimal amount)
    {
        if (amount > 0)
        {
            Balance += amount;
        }
        // else ignore or throw
    }
}

“Methods encode business rules. Properties hold data. Behavior protects invariants.”

3) “C# allows implementing only ONE interface” — False

Quiz version

“C# allows the implementation of only ONE interface.”

✅ Answer: False

Real rule (must‑memorize)

• ✅ A class can implement multiple interfaces.
• ❌ A class can inherit from only one base class.

public sealed class ExportService : IDisposable, IAsyncDisposable, IHostedService
{
    public void Dispose() { /*...*/ }
    public ValueTask DisposeAsync() => ValueTask.CompletedTask;
    public Task StartAsync(CancellationToken ct) => Task.CompletedTask;
    public Task StopAsync(CancellationToken ct) => Task.CompletedTask;
}

Interview soundbite

“Single inheritance for classes, multiple inheritance for interfaces.”

4) “How do we initialize properties during creation?” — Constructor (and friends)

Quiz version

“How can we initialize the properties of an object during its creation?”

✅ Answer: Using the constructor

Senior nuance: constructor vs. object initializer

Both can initialize values, but they serve different goals.

✅ Constructor = enforce invariants

Use it when an object must be created in a valid state.

public sealed class Order
{
    public Guid Id { get; }
    public string CustomerEmail { get; }

    public Order(Guid id, string customerEmail)
    {
        if (string.IsNullOrWhiteSpace(customerEmail))
            throw new ArgumentException("Email required.", nameof(customerEmail));

        Id = id;
        CustomerEmail = customerEmail;
    }
}

✅ Object initializer = ergonomics for optional fields

Great for DTOs or option bags.

var opts = new HttpClientOptions
{
    Timeout = TimeSpan.FromSeconds(10),
    Retries = 3
};

Interview soundbite

“Constructors establish invariants. Object initializers are ergonomic for optional configuration.”

5) “A class can be used as a property type in another class” — Composition

✅ Answer: True

This is composition: a has‑a relationship (often better than inheritance).

public sealed class Address
{
    public string Street { get; init; } = "";
}

public sealed class Person
{
    public Address Address { get; init; } = new();
}

Why seniors prefer composition

• Inheritance creates tight coupling.
• Composition keeps boundaries cleaner and enables substitution.

Interview soundbite

“Prefer composition: it’s easier to refactor and test than deep inheritance chains.”

6) “Access modifiers define the …” (content truncated in the original)

Quiz version

“Access modifiers define the visibility of members.”

✅ Answer: True

What access modifiers actually control

Senior perspective

• Encapsulation is about who can touch a member, not what exists.
• Over‑exposing members (public) can become a design smell.
• Prefer the most restrictive modifier that still satisfies the required usage.

public abstract class Shape
{
    // Only derived types can read/write the radius.
    protected double Radius { get; set; }

    // Only this class can change the ID after construction.
    public Guid Id { get; private set; } = Guid.NewGuid();

    // Internal helper used by the assembly but hidden from consumers.
    internal void Validate() { /*...*/ }
}

Interview soundbite

“Access modifiers are a contract about who may interact with a member, not a statement about the member’s existence.”

7) “Why do record types exist?” — Value‑equality & immutability

Quiz version

“What makes record types special in modern C#?”

✅ Answer: Value equality, built‑in immutability, and concise data modeling

Key benefits

• Value‑based Equals/GetHashCode generated automatically.
• with expressions for non‑destructive mutation.
• Concise syntax for DTOs and domain models.

public record PersonDto(string FirstName, string LastName);

var p1 = new PersonDto("Alice", "Smith");
var p2 = p1 with { LastName = "Johnson" }; // creates a new immutable instance
bool same = p1 == new PersonDto("Alice", "Smith"); // true – value equality

Interview soundbite

“Use record when you need immutable, value‑semantic data carriers; avoid them for entities with identity semantics.”

Closing thoughts

Understanding the why behind these OOP fundamentals lets you:

• Write code that enforces invariants and protects state.
• Choose the right inheritance vs. composition strategy.
• Communicate clearly in code reviews and interviews.

Master these mental models, and you’ll move from answering quiz questions to building robust, production‑grade .NET applications.

C# Interview Quick‑Check

1️⃣ “A class has properties and methods a class has” — False

Correct answer: False

Reality
Access modifiers define visibility, not existence.

public sealed class User
{
    public string Email { get; private set; } = "";
    private string PasswordHash { get; set; } = "";
}

Interview soundbite
“Access modifiers answer: ‘who can touch this member?’ not ‘does it exist?’”

2️⃣ “Can C# inherit from multiple classes?” — No

Correct answer: Only one class

public class Employee : Person // ✅
{
}

How we model “multiple inheritance” in C#

• Interfaces for capabilities
• Composition for behavior reuse
• Decorators and delegation for cross‑cutting concerns

Interview soundbite
“C# avoids multiple class inheritance—interfaces + composition give you flexibility without diamond problems.”

3️⃣ “What symbol assigns a value to a property?” — =

Correct answer: =

person.Name = "Juan"; // assignment operator

Seniors care because assignment is often where invariants get violated.
That’s why you frequently see:

• private set;
• init;
• methods to mutate safely

4️⃣ protected access scope — True (but use carefully)

Correct answer: True

protected means:

• accessible inside the declaring type
• accessible inside derived types

public class Base
{
    protected int Value;
}

public class Derived : Base
{
    public void Set() => Value = 10; // ✅ allowed
}

Senior warning
protected can create leaky inheritance designs where derived classes depend on fragile internal details.
Use it when:

• the base class is explicitly designed for extension
• you control the inheritance hierarchy
• you have a stable, documented template‑method model

5️⃣ Records matter because of value equality — True

Correct answer: True

Why record exists

Records are designed for data modeling:

• value‑based equality
• easy immutability
• structural comparison

Great for DDD messages / events / DTOs.

public record Person(string Name, int Age);

var p1 = new Person("Ana", 30);
var p2 = new Person("Ana", 30);

Console.WriteLine(p1 == p2); // ✅ true (value equality)

Interview soundbite
“Records are for data with value semantics—great for DTOs and domain events.”

6️⃣ Interface keyword — interface

public interface ILogger
{
    void Log(string message);
}

7️⃣ Class keyword — class

public class Order { }

8️⃣ “Base concepts of OOP” — Class and Object

Correct answer: class and object

Everything else (inheritance, polymorphism, abstraction, encapsulation) is built on that foundation.

9️⃣ Polymorphism in C# — Overriding (and interfaces)

Correct answer: Overwrite

public abstract class Animal
{
    public abstract string Speak();
}

public sealed class Dog : Animal
{
    public override string Speak() => "Bark";
}

🔟 Abstraction statement — True

Correct answer: True

Abstraction defines what to do, without specifying how:

• interfaces
• abstract classes

1️⃣1️⃣ “What to consider for a property?” — Access level, type, name

Correct answer: A. Access level, type and name

public string Name { get; private set; } = "";

1️⃣2️⃣ Encapsulation keywords — get; set;

Correct answer: get; set;

Senior upgrade
Encapsulation isn’t “public get/set everywhere”. It’s controlling mutation.

public sealed class Customer
{
    public string Email { get; private set; }

    public Customer(string email)
    {
        // validate and normalize
        Email = email.Trim().ToLowerInvariant();
    }

    public void ChangeEmail(string newEmail)
    {
        Email = newEmail.Trim().ToLowerInvariant();
    }
}

Final Thoughts — Why these “basic” answers matter

These questions are not trivia. They map directly to:

• SOLID (encapsulation, abstraction, substitutability)
• Clean Architecture (boundaries, composition)
• Dependency Injection (interfaces, object‑graph creation)
• Maintainability (invariants and controlled mutation)

If you can explain these mental models clearly, you’re not just “knowing C#” — you’re reasoning like an engineer who can build systems that survive change.

✍️ Written by Cristian Sifuentes — building resilient .NET systems, teaching teams how to reason about OOP, boundaries, and clean architecture.