Interview Q&A

Short answer: When you want to share common code among related classes. When you need fields or constructors. When future changes may require adding non-breaking methods.

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: Can you inherit multiple abstract classes? Yes → Multiple interfaces No → Multiple abstract classes (C# does not support multiple class inheritance)

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: bstract classes? Yes → Multiple interfaces No → Multiple abstract classes (C# does not support multiple class inheritance)

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: Yes → Multiple interfaces No → Multiple abstract classes (C# does not support multiple class inheritance)

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: Interface allows multiple inheritance. Interface allows multiple inheritance.

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: Yes, starting from C# 8, interfaces can have default method implementations. 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, abstract classes can have fully implemented methods along with abstract methods.

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, both can define properties. Interface properties are abstract by default; abstract class properties can have implementation. interface ICar { int Speed { get; set; } } abstract class Vehicle { public int Speed { get; set; } }

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, you cannot instantiate an interface. You can only use it as a reference type. ICar car = new Car(); // Interface reference // ICar c = new ICar(); // Not allowed

Example code

No, you cannot instantiate an interface. You can only use it as a reference type. ICar car = new Car(); // Interface reference
// ICar c = new ICar(); // Not allowed

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 be injected as service contracts, but interfaces are preferred for looser 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: Both can define contracts for derived classes. Both support polymorphism. Both cannot be instantiated directly. Both can be used with 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.

Short answer: Interface contracts are pure method signatures. Abstract class contracts can contain shared code and fields.

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 class must provide implementation once. Explicit interface implementation can resolve ambiguity. interface IDriveable { void Start(); } abstract class Vehicle { public abstract void Start(); } class Car : Vehicle, IDriveable

Example code

{
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: No, interfaces can only inherit other interfaces.

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: Yes, abstract classes can implement interfaces partially or fully.

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: bstract classes? Abstract classes are slightly faster because they use direct method calls. Interfaces may incur slight overhead due to indirect method calls via vtable. Difference is usually negligible in most applications.

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 are slightly faster because they use direct method calls. Interfaces may incur slight overhead due to indirect method calls via vtable. Difference is usually negligible in most applications.

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 are preferred for public APIs because they: Allow multiple inheritance Support loose coupling Avoid breaking changes when adding new implementations Abstract classes are better for internal APIs where shared code is required.

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: Adding new members breaks existing implementations unless using default interface methods (C# 8+). Abstract Classes: Can add new methods with implementation without breaking derived classes.

Example code

Interfaces: Adding new members breaks existing implementations unless using default interface methods (C# 8+). Abstract Classes: Can add new methods with implementation without breaking derived 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: Interfaces support multiple inheritance. Abstract classes do not. Interview Q&A

Example code

Interfaces support multiple inheritance. Abstract classes do not. Interview 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: Define an interface like IPlugin with a Run() method.

Explain a bit more

Each plugin implements IPlugin and can be loaded dynamically. interface IPlugin { void Run(); } class PluginA : IPlugin { public void Run() => Console.WriteLine("Plugin A running"); } class PluginB : IPlugin { public void Run() => Console.WriteLine("Plugin B running"); } // Usage List<IPlugin> plugins = new List<IPlugin> { new PluginA(), new PluginB() }; foreach (var p in plugins) p.Run();

Example code

Define an interface like IPlugin with a Run() method. Each plugin implements IPlugin and can be loaded dynamically. interface IPlugin { void Run(); }
class PluginA : IPlugin { public void Run() => Console.WriteLine("Plugin A running"); } class PluginB : IPlugin { public void Run() => Console.WriteLine("Plugin B running"); } // Usage List<IPlugin> plugins = new List<IPlugin> { new PluginA(), new PluginB() }; foreach (var p in plugins) p.Run();

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: And interfaces? Use an interface for common operations: IPayment. Use an abstract class for shared behavior like logging. interface IPayment { void Pay(decimal amount); } bstract class PaymentBase : IPayment { public abstract void Pay(decimal amount); public void Log(string message) => Console.WriteLine(message); } class CreditCardPayment :… PaymentBase {……… nd interfaces? nd interfaces? Use an interface for common…

Explain a bit more

operations: IPayment. Use an abstract class for shared behavior like logging. interface IPayment { void Pay(decimal amount); } bstract class PaymentBase : IPayment { public abstract void Pay(decimal amount); public void Log(string message) => Console.WriteLine(message); } class CreditCardPayment : PaymentBase {… nd interfaces? Use an interface for common operations: IPayment. Use an abstract class for shared behavior like logging. interface IPayment { void Pay(decimal amount); } bstract class PaymentBase : IPayment { public abstract void Pay(decimal amount); public void Log(string message) => Console.WriteLine(message); } class… And… public abstract void Pay(decimal amount);

Example code

public void Log(string message) => Console.WriteLine(message);
}
class CreditCardPayment : PaymentBase
{
public override void Pay(decimal amount) => Console.WriteLine($"Paid {amount} by Credit Card"); }

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 an interface for common operations: IPayment. Use an abstract class for shared behavior like logging. interface IPayment { void Pay(decimal amount); } abstract class PaymentBase : IPayment {

Example code

public abstract void Pay(decimal amount);
public void Log(string message) => Console.WriteLine(message);
}
class CreditCardPayment : PaymentBase
{
public override void Pay(decimal amount) => Console.WriteLine($"Paid {amount} by Credit Card"); }

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 a base Notification class or interface. Derive classes like EmailNotification, SMSNotification. abstract class Notification { public abstract void Send(string message); } class EmailNotification : Notification { public override void

Example code

Send(string msg) => Console.WriteLine("Email: " + msg); }
class SMSNotification : Notification { public override void
Send(string msg) => Console.WriteLine("SMS: " + msg); }
List<Notification> notifications = new List<Notification> { new EmailNotification(), new SMSNotification() }; foreach (var n in notifications) n.Send("Hello World!");

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: Protects internal data by restricting direct access. Ensures sensitive fields are accessed only via methods/properties, preventing misuse.

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.