Interview Q&A

Short answer: Yes, methods can have default implementations in interfaces. interface ILogger

Example code

{ void Log(string message); void LogWarning(string message) => Console.WriteLine("Warning: " + message); }

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 Interface Class Implementatio No implementation (except default methods) Can have full implementation Fields Cannot have fields Can have fields Instantiation Cannot instantiate Can instantiate Inheritance Can inherit multiple interfaces Single class inheritance only

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: Yes, a class can implement multiple interfaces, solving multiple inheritance issues. class FlyingCar : IDriveable, IFlyable

Example code

{
public void Drive() => Console.WriteLine("Driving");
public void Fly() => Console.WriteLine("Flying");
}

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: The implementing class must provide a single implementation for both interfaces. Or use explicit interface implementation to differentiate.

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: Yes, explicit implementation allows a class to implement interface members separately. class Car : IDriveable

Example code

{
void IDriveable.Drive() => Console.WriteLine("Explicit drive");
}

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: Implementing an interface member explicitly so it can only be called via interface reference, not class object. IDriveable car = new Car(); car.Drive(); // Works // Car c = new Car(); c.Drive(); // Won't compile

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: Define contracts for classes. Achieve abstraction, polymorphism, and loose coupling.

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: Expose method signatures without implementation. Users interact with the interface, not the underlying implementation.

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: Code depends on interface, not concrete class. Makes it easier to swap implementations without changing dependent code. void StartVehicle(IDriveable vehicle) { vehicle.Drive(); }

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 DI frameworks to inject concrete implementations at runtime. Promotes flexibility and testability. public class CarService { private readonly IDriveable _vehicle; public CarService(IDriveable vehicle) { _vehicle = vehicle; } }

Example code

Interfaces allow DI frameworks to inject concrete implementations at runtime. Promotes flexibility and testability. public class CarService
{
private readonly IDriveable _vehicle;
public CarService(IDriveable vehicle) { _vehicle = vehicle; }
}

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: Provides a standard method to compare objects for sorting. class Employee : IComparable<Employee>

Example code

{
public int Id { get; set; }
public int CompareTo(Employee other) => this.Id.CompareTo(other.Id); }

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: Provides Dispose() method for releasing unmanaged resources. class FileHandler : IDisposable

Example code

{
public void Dispose() => Console.WriteLine("Resources released"); }

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: IEnumerable → Provides collection traversal capability (GetEnumerator() method). IEnumerator → Used to iterate over a collection (MoveNext(), Current, Reset()).

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, interfaces can inherit from other interfaces, forming a hierarchy. interface IFlyable { void Fly(); } interface IAdvancedFlyable : IFlyable { void Loop(); }

Example code

Yes, interfaces can inherit from other interfaces, forming a hierarchy. interface IFlyable { void Fly(); }
interface IAdvancedFlyable : IFlyable { void Loop(); }

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: Interfaces with no methods or properties, used to mark classes for special behavior. Example: ISerializable marks classes as serializable.

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: An abstract class is a class that cannot be instantiated directly. Can contain abstract methods (without implementation) and concrete methods (with implementation). Used to define a common base for other classes.

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: Use the abstract keyword. abstract class Vehicle {

Example code

public abstract void Start();
public void Stop() => Console.WriteLine("Vehicle stopped");
}

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: A method declared with abstract without implementation. Must be overridden in a derived class. abstract class Vehicle {

Example code

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

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: Yes, abstract classes can have fields, properties, and constants. abstract class Vehicle { protected string Brand; }

Example code

Yes, abstract classes can have fields, properties, and constants. abstract class Vehicle { protected string Brand; }

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: Yes, abstract classes can implement interfaces partially or fully. Derived classes must implement any remaining abstract members. interface IDriveable { void Drive(); } abstract class Vehicle : IDriveable { public abstract void Drive(); }

Example code

class Car : Vehicle { public override void Drive() => Console.WriteLine("Car 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: No, abstract classes cannot be sealed. A sealed class cannot be inherited, while abstract classes are meant to be inherited.

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: Yes, abstract classes can have private members, but derived classes cannot access them. Private members can be accessed via protected or public methods.

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: No, a class cannot be both abstract and static. Abstract classes are for inheritance, static classes cannot be inherited.

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: llow derived class to optionally override

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 Abstract Method Virtual Method Implementatio No implementation Has implementation Must override? Must be overridden Optional to override Class type Must be in abstract class Can be in any class Purpose Force derived classes to implement Allow derived class to optionally override

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.