Interview Q&A

Short answer: Code depends on interfaces or base classes, not concrete implementations. Makes system flexible, extendable, and easier to maintain. void StartVehicle(Vehicle v) { v.Start(); } // Works with any derived type

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: Advantages: Promotes code reuse and flexibility Enables loose coupling Supports extensible architecture Disadvantages: May introduce runtime overhead Can make code harder to understand if overused Requires careful design to avoid ambiguity

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, interfaces cannot have fields. Only methods, properties, events, or indexers.

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, interfaces cannot have constructors.

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, starting from C# 8, interfaces can contain static methods. interface IUtility

Example code

{ static void Show() => Console.WriteLine("Static method in interface");
}

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, 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: 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: 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: 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: 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: 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: Abstract classes can provide shared implementation, fields, and constructors. Useful when multiple classes share common behavior along with enforced abstraction.

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 Employee {

Example code

public string Name { get; set; }
public abstract void Work();
public void Report() => Console.WriteLine("Reporting work done"); }
class Developer : Employee
{
public override void Work() => Console.WriteLine("Writing code"); }
class Tester : Employee
{
public override void Work() => Console.WriteLine("Testing application"); } // Usage Employee dev = new Developer() { Name = "Alice" }; dev.Work(); dev.Report();

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 methods must be overridden with override in derived classes. You can then mark the overriding method as virtual to allow further overriding in subclasses. abstract class Vehicle { public abstract void Start(); } class Car : Vehicle { public override void Start() => Console.WriteLine("Car starts"); }

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 allow multiple class inheritance. Use interfaces as a workaround. interface IFlyable { void Fly(); } interface IDriveable { void Drive(); } class FlyingCar : IFlyable, IDriveable { public void Fly() {} public void Drive() {} } Q&A

Example code

No, C# does not allow multiple class inheritance. Use interfaces as a workaround. interface IFlyable { void Fly(); }
interface IDriveable { void Drive(); }
class FlyingCar : IFlyable, IDriveable { public void Fly() {} public void Drive() {} } Q&A

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: When you want to define pure contracts without implementation. When you need multiple inheritance. When you want loose coupling for dependency injection.

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.