What is the difference between interface-based and abstract class-based abstraction?

Short answer: Feature Abstract Class Interface Methods Can have abstract + concrete methods Only abstract methods (C# 8+ allows default implementation) Fields Can have fields Cannot have fields Inheritance Single inheritance Multiple interfaces can be implemented Constructo Can have constructors Cannot have constructors

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.