Interview Q&A

Short answer: Using the colon (:) symbol. Derived class can access public/protected members of the base class. class Vehicle { public void Start() => Console.WriteLine("Start"); }

Example code

class Car : Vehicle { }
Car myCar = new Car(); myCar.Start(); // Inherited method

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: No, C# does not support multiple class inheritance to avoid ambiguity.

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: Common functionality is implemented in base class. Derived classes reuse the code without duplicating it, reducing maintenance effort.

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: No, private members are hidden from derived classes. Can access protected, internal, or public members. class Vehicle { private int id; protected string model; }

Example code

class Car : Vehicle { /* cannot access id, can access model */ }

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: Yes, constructors can have multiple signatures in the same class. class Car

Example code

{
public Car() { }
public Car(string model) { }
}

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: No, constructors cannot be inherited or overridden. Base class constructor can be called using : base(), but cannot be overridden.

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: How is polymorphism implemented in C#? is a common interview topic in C# OOP. Give a clear definition, then one concrete example.

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: abstract class Shape { public abstract void Draw(); } class Circle : Shape { public override void Draw() => Console.WriteLine("Drawing Circle"); } class Rectangle : Shape { public override void Draw() => Console.WriteLine("Drawing Rectangle"); } Shape s1 = new Circle();

Example code

Shape s2 = new Rectangle(); s1.Draw(); // Circle's Draw s2.Draw(); // Rectangle's Draw

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.