How do you mock dependencies in unit tests using DI?

• 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.