How would you design a payment system using abstract classes and interfaces?

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.