What is the Dependency Inversion Principle and how do interfaces support it?

Short answer: High-level modules should not depend on low-level modules. Both should depend on abstractions. Interfaces allow decoupling and easier testing. interface ILogger { void Log(string message); }

Example code

class FileLogger : ILogger { public void Log(string message) => Console.WriteLine("File: " + message); } class UserService
{
private readonly ILogger _logger;
public UserService(ILogger logger) { _logger = logger; }
}

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.