How do you mock external service calls in unit tests?

Short answer: Use mocking frameworks like Moq to create mock implementations of service interfaces. For HTTP calls, tools like HttpClientFactory with a mocked HttpMessageHandler or libraries like RichardSzalay.MockHttp help simulate HTTP responses.

Real-world example (ShopNest)

When testing ShopNest’s order service, mock IPaymentGateway so tests do not call Razorpay.

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.