What is the key difference between abstract classes and interfaces?

Short answer: Feature Abstract Class Interface Implementation Can have full/partial implementation Cannot have full implementation (except default methods in C# 8+) Fields Can have fields Cannot have fields Inheritance Single class inheritance Multiple interface inheritance allowed Constructors Allowed Not allowed Access Modifiers Can have public, protected, private Members are public by default

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.