migration-unit-testing

Migration Unit Testing Skill

Use this skill when creating unit tests to validate migrated applications work correctly after modernization.

When to Use This Skill

• Creating tests to validate migration correctness
• Building test suites for migrated applications of any stack (.NET, Java, Python, Node.js, PHP, Ruby, Go, etc.). This skill ships worked examples for .NET and Java; adapt the same patterns to other stacks using their idiomatic test frameworks (pytest for Python, jest/vitest for Node, phpunit for PHP, rspec for Ruby, testing package for Go, etc.).
• Implementing equivalence testing (old vs new behavior)
• Setting up mocking for external dependencies
• Creating regression tests for business logic
• Establishing test coverage baselines

Testing Strategy for Migrated Applications

Priority Order

1. Business Logic - Critical calculations, validations, workflows
2. Data Access - Repository operations, query correctness
3. API Endpoints - Request/response contracts, status codes
4. Authentication/Authorization - Security flows
5. Integrations - External service interactions
6. UI Components - View models, presentation logic

Coverage Goals

.NET Testing Patterns (xUnit)

Project Setup

<!-- Tests.csproj -->
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
    <IsPackable>false</IsPackable>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
    <PackageReference Include="xunit" Version="2.6.2" />
    <PackageReference Include="xunit.runner.visualstudio" Version="2.5.4" />
    <PackageReference Include="Moq" Version="4.20.70" />
    <PackageReference Include="FluentAssertions" Version="6.12.0" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="10.0.0" />
    <PackageReference Include="Microsoft.EntityFrameworkCore.InMemory" Version="10.0.0" />
  </ItemGroup>
</Project>

Test Naming Convention

MethodName_Scenario_ExpectedBehavior

Examples:

• GetUser_WithValidId_ReturnsUser
• CreateOrder_WithInvalidItems_ThrowsValidationException
• CalculateDiscount_WhenTotalExceeds100_Returns10Percent

Service Layer Test

public class UserServiceTests
{
    private readonly Mock<IUserRepository> _repositoryMock;
    private readonly Mock<ILogger<UserService>> _loggerMock;
    private readonly UserService _sut; // System Under Test
    
    public UserServiceTests()
    {
        _repositoryMock = new Mock<IUserRepository>();
        _loggerMock = new Mock<ILogger<UserService>>();
        _sut = new UserService(_repositoryMock.Object, _loggerMock.Object);
    }
    
    [Fact]
    public async Task GetByIdAsync_WithExistingUser_ReturnsUser()
    {
        // Arrange
        var expectedUser = new User { Id = 1, Name = "John Doe", Email = "[email protected]" };
        _repositoryMock
            .Setup(r => r.GetByIdAsync(1))
            .ReturnsAsync(expectedUser);
        
        // Act
        var result = await _sut.GetByIdAsync(1);
        
        // Assert
        result.Should().NotBeNull();
        result.Should().BeEquivalentTo(expectedUser);
    }
    
    [Fact]
    public async Task GetByIdAsync_WithNonExistingUser_ReturnsNull()
    {
        // Arrange
        _repositoryMock
            .Setup(r => r.GetByIdAsync(It.IsAny<int>()))
            .ReturnsAsync((User?)null);
        
        // Act
        var result = await _sut.GetByIdAsync(999);
        
        // Assert
        result.Should().BeNull();
    }
    
    [Theory]
    [InlineData("")]
    [InlineData(" ")]
    [InlineData(null)]
    public async Task CreateAsync_WithInvalidName_ThrowsArgumentException(string? invalidName)
    {
        // Arrange
        var dto = new CreateUserDto(invalidName!, "[email protected]");
        
        // Act
        var act = () => _sut.CreateAsync(dto);
        
        // Assert
        await act.Should().ThrowAsync<ArgumentException>()
            .WithMessage("*name*");
    }
}

Controller Integration Test

public class UsersControllerTests : IClassFixture<WebApplicationFactory<Program>>
{
    private readonly HttpClient _client;
    private readonly WebApplicationFactory<Program> _factory;
    
    public UsersControllerTests(WebApplicationFactory<Program> factory)
    {
        _factory = factory.WithWebHostBuilder(builder =>
        {
            builder.ConfigureServices(services =>
            {
                // Replace real DB with in-memory
                var descriptor = services.SingleOrDefault(
                    d => d.ServiceType == typeof(DbContextOptions<AppDbContext>));
                if (descriptor != null)
                    services.Remove(descriptor);
                
                services.AddDbContext<AppDbContext>(options =>
                    options.UseInMemoryDatabase("TestDb"));
            });
        });
        _client = _factory.CreateClient();
    }
    
    [Fact]
    public async Task GetUsers_ReturnsSuccessAndCorrectContentType()
    {
        // Act
        var response = await _client.GetAsync("/api/users");
        
        // Assert
        response.EnsureSuccessStatusCode();
        response.Content.Headers.ContentType?.MediaType.Should().Be("application/json");
    }
    
    [Fact]
    public async Task GetUser_WithInvalidId_ReturnsNotFound()
    {
        // Act
        var response = await _client.GetAsync("/api/users/99999");
        
        // Assert
        response.StatusCode.Should().Be(HttpStatusCode.NotFound);
    }
    
    [Fact]
    public async Task CreateUser_WithValidData_ReturnsCreatedWithLocation()
    {
        // Arrange
        var newUser = new { Name = "Jane Doe", Email = "[email protected]" };
        var content = new StringContent(
            JsonSerializer.Serialize(newUser),
            Encoding.UTF8,
            "application/json");
        
        // Act
        var response = await _client.PostAsync("/api/users", content);
        
        // Assert
        response.StatusCode.Should().Be(HttpStatusCode.Created);
        response.Headers.Location.Should().NotBeNull();
    }
}

Database Test with In-Memory Provider

public class UserRepositoryTests : IDisposable
{
    private readonly AppDbContext _context;
    private readonly UserRepository _sut;
    
    public UserRepositoryTests()
    {
        var options = new DbContextOptionsBuilder<AppDbContext>()
            .UseInMemoryDatabase(Guid.NewGuid().ToString())
            .Options;
        _context = new AppDbContext(options);
        _sut = new UserRepository(_context);
    }
    
    [Fact]
    public async Task AddAsync_AddsUserToDatabase()
    {
        // Arrange
        var user = new User { Name = "Test User", Email = "[email protected]" };
        
        // Act
        await _sut.AddAsync(user);
        await _context.SaveChangesAsync();
        
        // Assert
        var savedUser = await _context.Users.FirstOrDefaultAsync(u => u.Email == "[email protected]");
        savedUser.Should().NotBeNull();
        savedUser!.Name.Should().Be("Test User");
    }
    
    public void Dispose() => _context.Dispose();
}

Java Testing Patterns (JUnit 5)

Project Setup (pom.xml)

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.mockito</groupId>
        <artifactId>mockito-junit-jupiter</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.assertj</groupId>
        <artifactId>assertj-core</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>com.h2database</groupId>
        <artifactId>h2</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>

Service Layer Test

@ExtendWith(MockitoExtension.class)
class UserServiceTest {
    
    @Mock
    private UserRepository userRepository;
    
    @InjectMocks
    private UserService userService;
    
    @Test
    @DisplayName("Should return user when valid ID provided")
    void findById_WithValidId_ReturnsUser() {
        // Given
        User expectedUser = new User(1L, "John Doe", "[email protected]");
        when(userRepository.findById(1L)).thenReturn(Optional.of(expectedUser));
        
        // When
        Optional<User> result = userService.findById(1L);
        
        // Then
        assertThat(result)
            .isPresent()
            .hasValueSatisfying(user -> {
                assertThat(user.getName()).isEqualTo("John Doe");
                assertThat(user.getEmail()).isEqualTo("[email protected]");
            });
    }
    
    @Test
    @DisplayName("Should return empty when user not found")
    void findById_WithNonExistingId_ReturnsEmpty() {
        // Given
        when(userRepository.findById(anyLong())).thenReturn(Optional.empty());
        
        // When
        Optional<User> result = userService.findById(999L);
        
        // Then
        assertThat(result).isEmpty();
    }
    
    @ParameterizedTest
    @NullAndEmptySource
    @ValueSource(strings = {" ", "   "})
    @DisplayName("Should throw exception for invalid name")
    void create_WithInvalidName_ThrowsException(String invalidName) {
        // Given
        CreateUserDto dto = new CreateUserDto(invalidName, "[email protected]");
        
        // When/Then
        assertThatThrownBy(() -> userService.create(dto))
            .isInstanceOf(IllegalArgumentException.class)
            .hasMessageContain

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