Interview Q&A

Short answer: 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;

Example code

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

Real-world example (ShopNest)

ShopNest’s Order keeps _items private and exposes AddItem() so totals stay correct—callers cannot put a negative quantity directly into the list.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Private → Hides data from outside access, ensuring security. Public → Provides controlled access through properties or methods. Example: private decimal balance; // hidden

Example code

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

Real-world example (ShopNest)

ShopNest’s Order keeps _items private and exposes AddItem() so totals stay correct—callers cannot put a negative quantity directly into the list.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: 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 code

protected string accountType; // accessible in derived classes internal string branchCode; // accessible within same assembly

Real-world example (ShopNest)

Think of ShopNest’s Product, Cart, and Order classes: each object holds data + behavior, so pricing rules stay next to the data they use.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Properties provide controlled access to private fields. Enable validation, read-only/write-only access, and future flexibility. Example: private int score;

Example code

public int Score
{ get { return score; } set { if (value >= 0) score = value; } // validation
}

Real-world example (ShopNest)

ShopNest’s Order keeps _items private and exposes AddItem() so totals stay correct—callers cannot put a negative quantity directly into the list.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: 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 code

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

Real-world example (ShopNest)

Checkout calls IPaymentGateway.Charge(). Razorpay and Stripe adapters implement the same interface, so ShopNest can switch gateways without rewriting the order service.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: 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

Real-world example (ShopNest)

ShopNest has a base PaymentMethod with virtual decimal Fee(). UpiPayment and CardPayment override the fee logic without changing the checkout caller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

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

Real-world example (ShopNest)

Checkout calls IPaymentGateway.Charge(). Razorpay and Stripe adapters implement the same interface, so ShopNest can switch gateways without rewriting the order service.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

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

Example code

class Car : Vehicle {} // Derived

Real-world example (ShopNest)

ShopNest has a base PaymentMethod with virtual decimal Fee(). UpiPayment and CardPayment override the fee logic without changing the checkout caller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: 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() {} }

Example code

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() {} }

Real-world example (ShopNest)

ShopNest has a base PaymentMethod with virtual decimal Fee(). UpiPayment and CardPayment override the fee logic without changing the checkout caller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short 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

Real-world example (ShopNest)

ShopNest has a base PaymentMethod with virtual decimal Fee(). UpiPayment and CardPayment override the fee logic without changing the checkout caller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: A derived class provides a new implementation of a base class method marked virtual. Achieves run-time polymorphism. class Vehicle { public virtual void Start() => Console.WriteLine("Vehicle starts"); } class Car : Vehicle { public override void Start() => Console.WriteLine("Car starts"); }

Real-world example (ShopNest)

Think of ShopNest’s Product, Cart, and Order classes: each object holds data + behavior, so pricing rules stay next to the data they use.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

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

Real-world example (ShopNest)

ShopNest has a base PaymentMethod with virtual decimal Fee(). UpiPayment and CardPayment override the fee logic without changing the checkout caller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

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

Real-world example (ShopNest)

Think of ShopNest’s Product, Cart, and Order classes: each object holds data + behavior, so pricing rules stay next to the data they use.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: 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");

Example code

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

Real-world example (ShopNest)

ShopNest has a base PaymentMethod with virtual decimal Fee(). UpiPayment and CardPayment override the fee logic without changing the checkout caller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

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

Real-world example (ShopNest)

ShopNest has a base PaymentMethod with virtual decimal Fee(). UpiPayment and CardPayment override the fee logic without changing the checkout caller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short 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.

Real-world example (ShopNest)

Checkout calls IPaymentGateway.Charge(). Razorpay and Stripe adapters implement the same interface, so ShopNest can switch gateways without rewriting the order service.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Also called static polymorphism. Resolved at compile time. Achieved through method overloading or operator overloading. class Calculator

Example code

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

Real-world example (ShopNest)

Checkout calls IPaymentGateway.Charge(). Razorpay and Stripe adapters implement the same interface, so ShopNest can switch gateways without rewriting the order service.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: 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

Real-world example (ShopNest)

Checkout calls IPaymentGateway.Charge(). Razorpay and Stripe adapters implement the same interface, so ShopNest can switch gateways without rewriting the order service.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Same method name with different parameters in the same class. Enables compile-time polymorphism. class MathHelper

Example code

{
public int Multiply(int a, int b) => a * b;
public int Multiply(int a, int b, int c) => a * b * c; // Overloaded }

Real-world example (ShopNest)

Think of ShopNest’s Product, Cart, and Order classes: each object holds data + behavior, so pricing rules stay next to the data they use.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Defining custom behavior for operators (+, -, *, etc.) for a class. class Point

Example code

{
public int X, Y;
public static Point operator +(Point a, Point b) => new Point {
X = a.X + b.X, Y = a.Y + b.Y };
}

Real-world example (ShopNest)

Think of ShopNest’s Product, Cart, and Order classes: each object holds data + behavior, so pricing rules stay next to the data they use.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short 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

Real-world example (ShopNest)

Think of ShopNest’s Product, Cart, and Order classes: each object holds data + behavior, so pricing rules stay next to the data they use.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

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

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short 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();

Example code

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

Real-world example (ShopNest)

Checkout calls IPaymentGateway.Charge(). Razorpay and Stripe adapters implement the same interface, so ShopNest can switch gateways without rewriting the order service.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: 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"); }

Example code

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"); }

Real-world example (ShopNest)

Checkout calls IPaymentGateway.Charge(). Razorpay and Stripe adapters implement the same interface, so ShopNest can switch gateways without rewriting the order service.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Another term for runtime polymorphism, achieved via method overriding.

Real-world example (ShopNest)

Checkout calls IPaymentGateway.Charge(). Razorpay and Stripe adapters implement the same interface, so ShopNest can switch gateways without rewriting the order service.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.