How do you mock interfaces in unit testing?

Short answer: Use mocking frameworks like Moq or NSubstitute. Provides fake implementations to test dependent classes. var mockLogger = new Mock<ILogger>(); mockLogger.Setup(x => x.Log(It.IsAny<string>()));

Example code

Use mocking frameworks like Moq or NSubstitute. Provides fake implementations to test dependent classes. var mockLogger = new Mock<ILogger>();
mockLogger.Setup(x => x.Log(It.IsAny<string>()));

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.