Professional Unit Testing

Testing is not an "Afterthought." It is the safety net that allows you to refactor your code without fear. A professional .NET architect uses the STACK: xUnit for runner, Moq for isolation, and FluentAssertions for readability.

1. AAA Pattern (Arrange, Act, Assert)

Every test must follow this structure. It makes tests predictable and easy to read. - **Arrange:** Set up the mocks and data. - **Act:** Execute the method under test. - **Assert:** Verify the result. **Architect Tip:** If your 'Arrange' section is 50 lines long, your class is too complex and violates the Single Responsibility Principle.

2. The Power of Moq

Moq allows you to "Fake" your dependencies. You can tell a mock database: "When I call `GetById(1)`, return a null object." This allows you to test how your business logic handles a 'Not Found' error without needing a real database.

3. FluentAssertions

Compare these two:

• `Assert.Equal(10, result.Count)` (Boring)
• `result.Count.Should().Be(10).And.NotBeNull()` (Professional)

FluentAssertions reads like English and provides much more detailed error messages when a test fails.

4. Interview Mastery

Q: "What is the difference between a 'Stub', a 'Mock', and a 'Spy'?"

Architect Answer: "A **Stub** is a simple fake that returns pre-defined data. A **Mock** is smarter—it remembers if a method was called and with what parameters. A **Spy** is a real object that you 'watch' to see how it was used. In modern .NET development, we use Moq for both Stubs and Mocks, focusing on 'Behavioral Verification'—proving that our code interacted with its dependencies correctly."