How are interfaces used in dependency injection?

Short answer: Interfaces allow DI frameworks to inject concrete implementations at runtime. Promotes flexibility and testability. public class CarService { private readonly IDriveable _vehicle; public CarService(IDriveable vehicle) { _vehicle = vehicle; } }

Example code

Interfaces allow DI frameworks to inject concrete implementations at runtime. Promotes flexibility and testability. public class CarService
{
private readonly IDriveable _vehicle;
public CarService(IDriveable vehicle) { _vehicle = vehicle; }
}

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.