What are abstract classes?

Short answer: Classes that cannot be instantiated directly and may contain abstract methods (without implementation). Can have fields, constructors, and concrete methods. abstract class Animal { public abstract void MakeSound(); public void Sleep() => Console.WriteLine("Sleeping"); }

Example code

Classes that cannot be instantiated directly and may contain abstract methods (without implementation). Can have fields, constructors, and concrete methods. abstract class Animal {
public abstract void MakeSound();
public void Sleep() => Console.WriteLine("Sleeping");
}

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.