Test Generator Skill

🎯 Use Case Demo

Scenario

Developer Mike just wrote a new BookingService with 5 methods. He needs unit tests but testing takes forever to write.

Before (Manual):

// Spend 30 minutes writing one test...
[Fact]
public void CreateBooking_ValidData_ReturnsBooking()
{
    // Arrange... Act... Assert... 😴
}

After (With Skill):

"Generate tests for BookingService.cs"

public class BookingServiceTests
{
    private readonly Mock<IFlightRepository> _flightRepo;
    private readonly Mock<IBookingRepository> _bookingRepo;
    private readonly BookingService _sut;

    public BookingServiceTests()
    {
        _flightRepo = new Mock<IFlightRepository>();
        _bookingRepo = new Mock<IBookingRepository>();
        _sut = new BookingService(_flightRepo.Object, _bookingRepo.Object);
    }

    [Fact]
    public void CreateBooking_WithValidFlight_ReturnsNewBooking()
    {
        // Arrange
        var flight = new Flight { Id = 1, AvailableSeats = 10 };
        _flightRepo.Setup(r => r.GetById(1)).Returns(flight);
        
        // Act
        var result = _sut.CreateBooking(1, "John Doe", "john@example.com");
        
        // Assert
        Assert.NotNull(result);
        Assert.Equal("John Doe", result.PassengerName);
    }

    [Fact]
    public void CreateBooking_FlightNotFound_ThrowsException()
    {
        // Arrange
        _flightRepo.Setup(r => r.GetById(99)).Returns((Flight)null);
        
        // Act & Assert
        Assert.Throws<FlightNotFoundException>(() => 
            _sut.CreateBooking(99, "John", "john@example.com"));
    }

    [Theory]
    [InlineData("")]
    [InlineData(null)]
    [InlineData("   ")]
    public void CreateBooking_InvalidPassengerName_ThrowsValidationException(string name)
    {
        // Arrange & Act & Assert
        Assert.Throws<ValidationException>(() => 
            _sut.CreateBooking(1, name, "john@example.com"));
    }

    // ... 10 more comprehensive tests generated
}

Time Saved: 2-3 hours per service class

---

When to Use This Skill

• After writing new code
• When improving test coverage
• Before submitting PRs
• For TDD (write tests first)

Test Types Generated

Type	Purpose	Example
Happy Path	Normal operation	Valid input → expected output
Edge Cases	Boundary conditions	Empty, null, max values
Error Cases	Exception handling	Invalid input → proper exception
Integration	Component interaction	Service → Repository → DB

Agent Instructions

When user asks for tests:

1. Read the source file to understand:

   • Class structure and dependencies
   • Public methods to test
   • Input/output types
   • Possible exceptions

2. Identify test scenarios:

   • Happy path for each method
   • Null/empty inputs
   • Boundary values
   • Exception conditions

3. Generate test class:

   • Setup with mocks
   • Arrange-Act-Assert pattern
   • Descriptive test names
   • Theory/InlineData for parameterized tests

4. Match project framework:

   • .NET → xUnit + Moq
   • Node.js → Jest
   • Python → pytest

Example Prompts

User: "Write tests for BookingService" → Generate comprehensive test file

User: "Add edge case tests for CreateBooking method" → Focus on specific method edge cases

User: "What scenarios should I test?" → List test cases without code

Demo Script

# 1. Show existing service
cat Services/BookingService.cs

# 2. Ask agent: "Generate unit tests for BookingService"

# 3. Agent creates BookingServiceTests.cs with:
#    - 15+ test methods
#    - Mocked dependencies
#    - Edge cases covered

# 4. Run tests
dotnet test

Benefits

Metric	Before	After	Improvement
Test writing time	2-3 hours	5 minutes	⬇️ 95%
Test coverage	30-50%	80-90%	⬆️ 2x
Edge cases found	Often missed	Comprehensive	✅ Complete
TDD adoption	Difficult	Easy	✅ Enabled