Orchestration Testing

Testing the Application Layer requires isolation. We need to verify that the 'Service' coordinates the domain and infrastructure correctly.

1. Mocking the Outside World

When testing a Use Case (like CreateOrderHandler), you mock the IOrderRepository and IEmailService. You check that the handler calls repository.Add() and then unitOfWork.SaveChangesAsync(). This verifies that the 'Recipe' of the use case is correct without needing a real database or a real email server.

2. MediatR Pipeline Testing

If you're using MediatR Behaviors for logging or validation, you can test these in isolation too. This ensures your cross-cutting concerns are working globally without having to repeat their tests for every single handler.

3. Architect Insight

Q: "Moq or NSubstitute?"

Architect Answer: "Both are excellent. **NSubstitute** has a cleaner, more readable syntax (e.g., repo.Received().Add(item)) while **Moq** is the veteran with more advanced features. For Clean Architecture, readability is king, so NSubstitute is often the preferred choice for senior .NET teams today."