Interview Q&A

• Encapsulation is the mechanism of hiding internal details of an object and

exposing only necessary functionalities.

• It helps in protecting data and maintaining control over how it is accessed or

modified.

Example: A BankAccount class hides its balance and only allows deposit/withdraw

operations:

private decimal balance;
public void Deposit(decimal amount) { if(amount > 0) balance +=

mount; }

• Private → Hides data from outside access, ensuring security.
• Public → Provides controlled access through properties or methods.

Example:

private decimal balance; // hidden
public decimal Balance { get { return balance; } } // read-only

ccess

• Internal → Accessible only within the same assembly.
• Protected → Accessible in the class and derived classes.
• Protected Internal → Accessible in derived classes or within the same assembly.

Example:

protected string accountType; // accessible in derived classes

internal string branchCode; 	// accessible within same assembly

• Properties provide controlled access to private fields.
• Enable validation, read-only/write-only access, and future flexibility.

Example:

private int score;
public int Score
{

get { return score; }

set { if (value >= 0) score = value; } // validation
}

• Abstraction is the process of hiding the internal implementation details of a

system and exposing only the essential features.

• It allows developers to focus on what an object does, not how it does it.

Example: A Vehicle class exposes Start() method without revealing engine details.

Feature Abstract Class Interface

Methods Can have abstract +

concrete methods

Only abstract methods (C# 8+ allows default

implementation)

Fields Can have fields Cannot have fields

Inheritance Single inheritance Multiple interfaces can be implemented

Constructo

Can have constructors Cannot have constructors

Answer: Reduces system complexity by focusing on essential features. Decouples modules, making large systems easier to maintain and extend. Promotes code reuse and flexibility.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Base Class (Parent) → Class whose members are inherited. Derived Class (Child) → Class that inherits from base class. class Vehicle { public void Start() {} } // Base class Car : Vehicle {} // Derived

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

• Calls the constructor of the base class from a derived class.
• Ensures base class initialization before derived class constructor runs.

class Vehicle { public Vehicle(string brand) {

Console.WriteLine(brand); } }

class Car : Vehicle
{
public Car(string brand) : base(brand) { Console.WriteLine("Car

created"); }

}

• Use interfaces to achieve multiple inheritance.
• A class can implement multiple interfaces.

interface IFlyable { void Fly(); }
interface IDriveable { void Drive(); }
class FlyingCar : IFlyable, IDriveable { public void Fly() {} public

void Drive() {} }

Answer: Feature Inheritance Composition Relationship "is-a" "has-a" Reuse Derived class reuses base class Object contains other objects Flexibility Less flexible More flexible Example: Inheritance → Car is a Vehicle Composition → Car has a Engine

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

• Derived class provides a new implementation for a virtual method in base class.
• Enables runtime polymorphism.

class Vehicle { public virtual void Start() =>

Console.WriteLine("Vehicle starts"); }

class Car : Vehicle { public override void Start() =>

Console.WriteLine("Car starts"); }

Answer: virtual → Marks a base class method as overridable. override → Overrides a virtual method in the derived class.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Prevents a class from being inherited or a method from being overridden. sealed class FinalClass { } class Car : FinalClass { } // Not allowed

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

• override → Overrides a virtual method in base class (runtime polymorphism).
• new → Hides a base class method (compile-time hiding, not true overriding).

class Vehicle { public void Start() => Console.WriteLine("Vehicle");
}
class Car : Vehicle { public new void Start() =>

Console.WriteLine("Car"); }

Answer: Base class constructor executes first, then derived class constructor. Ensures base members are initialized before derived members.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Polymorphism means “many forms”. It allows objects of different types to be treated as objects of a common base type. Achieved through method overloading, overriding, and interfaces.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

• Also called static polymorphism.
• Resolved at compile time.
• Achieved through method overloading or operator overloading.

class Calculator
{
public int Add(int a, int b) => a + b;
public double Add(double a, double b) => a + b; // Overloaded

method

}

• Also called dynamic polymorphism.
• Resolved at runtime using method overriding.

class Vehicle { public virtual void Start() =>

Console.WriteLine("Vehicle starts"); }

class Car : Vehicle { public override void Start() =>

Console.WriteLine("Car starts"); }

Vehicle v = new Car();

v.Start(); // Calls Car's Start() at runtime

Answer: Same method name with different parameters in the same class. Enables compile-time polymorphism. class MathHelper { public int Multiply(int a, int b) => a * b; public int Multiply(int a, int b, int c) => a * b * c; // Overloaded }

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Defining custom behavior for operators (+, -, *, etc.) for a class. class Point { public int X, Y; public static Point operator +(Point a, Point b) => new Point { X = a.X + b.X, Y = a.Y + b.Y }; }

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Feature Overloading Overriding Compile/Runtime Compile-time Runtime Same signature? No, different parameters Same signature Virtual required? No Yes Inheritance required? Not required Required

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Early binding → Resolved at compile time (e.g., method overloading). Late binding → Resolved at runtime (e.g., method overriding with virtual/override).

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: object is the base class for all C# types. Enables polymorphism, as any object can be referred using object type. object obj = new Car();

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

• Interfaces allow different classes to implement the same contract, enabling

dynamic behavior at runtime.

interface IDriveable { void Drive(); }
class Car : IDriveable { public void Drive() =>

Console.WriteLine("Car drives"); }

class Bike : IDriveable { public void Drive() =>

Console.WriteLine("Bike drives"); }