How does versioning work differently for interfaces and abstract classes?

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.