How do you mock dependencies in unit tests using DI?

Short answer: Use mocking libraries like Moq, NSubstitute, or FakeItEasy. Create mocks of interfaces and inject them into the class under test: var mockRepo = new Mock<IProductRepository>(); mockRepo.Setup(repo => repo.GetAll()).Returns(new List<Product> { ... }); var service = new ProductService(mockRepo.Object); // Act & Assert This allows testing in isolation without hitting real databases or external services.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

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.