Abstract Class vs Interface?

Short answer: Interface Meaning Defines capability and behavior contract Use when: Multiple inheritance is required System uses plug-in extensibility Loose coupling is necessary public interface IPayment { void Pay(decimal amount); } Abstract Class Meaning Provides base behavior with shared implementation Represents IS-A inheritance relationship public abstract class PaymentBase

Example code

{
public void Log() => Console.WriteLine("Payment logged");
public abstract void Pay(decimal amount);
} Summary Interface = Capability Abstract Class = Shared Base Behavior

Real-world example (ShopNest)

Answer with: definition → one ShopNest-style story → trade-off → how you would verify it in production.

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.