Source: kevintsengtw/dotnet-testing-agent-skills (MIT). Ported into dotnet-agent-harness.

Web API Integration Testing

Applicable Scenarios

Skill Level: Advanced
Prerequisites: xUnit basics, ASP.NET Core basics, Testcontainers basics, Clean Architecture
Estimated Learning Time: 60-90 minutes

Learning Objectives

After completing this skill, you will be able to:

1. Establish complete Web API integration testing architecture
2. Implement modern exception handling using IExceptionHandler
3. Validate standard ProblemDetails and ValidationProblemDetails format
4. Use Flurl to simplify URL construction for HTTP testing
5. Use AwesomeAssertions for precise HTTP response validation
6. Establish multi-container (PostgreSQL + Redis) testing environment

Core Concepts

IExceptionHandler - Modern Exception Handling

The IExceptionHandler interface introduced in ASP.NET Core 8+ provides a more elegant error handling approach than traditional middleware:

/// <summary>
/// Global exception handler
/// </summary>
public class GlobalExceptionHandler : IExceptionHandler
{
    private readonly ILogger<GlobalExceptionHandler> _logger;

    public GlobalExceptionHandler(ILogger<GlobalExceptionHandler> logger)
    {
        _logger = logger;
    }

    public async ValueTask<bool> TryHandleAsync(
        HttpContext httpContext,
        Exception exception,
        CancellationToken cancellationToken)
    {
        _logger.LogError(exception, "Unhandled exception occurred: {Message}", exception.Message);

        var problemDetails = CreateProblemDetails(exception);

        httpContext.Response.StatusCode = problemDetails.Status ?? 500;
        httpContext.Response.ContentType = "application/problem+json";

        await httpContext.Response.WriteAsJsonAsync(problemDetails, cancellationToken);
        return true;
    }

    private static ProblemDetails CreateProblemDetails(Exception exception)
    {
        return exception switch
        {
            KeyNotFoundException => new ProblemDetails
            {
                Type = "https://httpstatuses.com/404",
                Title = "Resource Not Found",
                Status = 404,
                Detail = exception.Message
            },
            ArgumentException => new ProblemDetails
            {
                Type = "https://httpstatuses.com/400",
                Title = "Invalid Parameters",
                Status = 400,
                Detail = exception.Message
            },
            _ => new ProblemDetails
            {
                Type = "https://httpstatuses.com/500",
                Title = "Internal Server Error",
                Status = 500,
                Detail = "An unexpected error occurred"
            }
        };
    }
}
```text

### ProblemDetails Standard Format

RFC 7807 defined unified error response format:

| Field | Description |
| ----- | ----------- |
| `type` | URI for problem type |
| `title` | Short error description |
| `status` | HTTP status code |
| `detail` | Detailed error explanation |
| `instance` | URI of problem occurrence |

### ValidationProblemDetails - Validation Error Specific

```json
{
  "type": "https://tools.ietf.org/html/rfc9110#section-15.5.1",
  "title": "One or more validation errors occurred.",
  "status": 400,
  "detail": "Input data contains validation errors",
  "errors": {
    "Name": ["Product name cannot be empty"],
    "Price": ["Product price must be greater than 0"]
  }
}
```text

### FluentValidation Exception Handler

FluentValidation exception handler implements the `IExceptionHandler` interface, specifically handling `ValidationException` and converting validation errors to standard `ValidationProblemDetails` format response. Handlers execute in registration order, and specific handlers (like FluentValidation) must be registered before global handlers.

> 📖 Complete implementation code please refer to [references/exception-handler-details.md](references/exception-handler-details.md)

## Integration Testing Infrastructure

### TestWebApplicationFactory

```csharp
public class TestWebApplicationFactory : WebApplicationFactory<Program>, IAsyncLifetime
{
    private PostgreSqlContainer? _postgresContainer;
    private RedisContainer? _redisContainer;
    private FakeTimeProvider? _timeProvider;

    public PostgreSqlContainer PostgresContainer => _postgresContainer
        ?? throw new InvalidOperationException("PostgreSQL container has not been initialized");

    public RedisContainer RedisContainer => _redisContainer
        ?? throw new InvalidOperationException("Redis container has not been initialized");

    public FakeTimeProvider TimeProvider => _timeProvider
        ?? throw new InvalidOperationException("TimeProvider has not been initialized");

    public async Task InitializeAsync()
    {
        _postgresContainer = new PostgreSqlBuilder()
            .WithImage("postgres:16-alpine")
            .WithDatabase("test_db")
            .WithUsername("testuser")
            .WithPassword("testpass")
            .WithCleanUp(true)
            .Build();

        _redisContainer = new RedisBuilder()
            .WithImage("redis:7-alpine")
            .WithCleanUp(true)
            .Build();

        _timeProvider = new FakeTimeProvider(new DateTimeOffset(2024, 1, 1, 0, 0, 0, TimeSpan.Zero));

        await _postgresContainer.StartAsync();
        await _redisContainer.StartAsync();
    }

    protected override void ConfigureWebHost(IWebHostBuilder builder)
    {
        builder.ConfigureAppConfiguration(config =>
        {
            config.Sources.Clear();
            config.AddInMemoryCollection(new Dictionary<string, string?>
            {
                ["ConnectionStrings:DefaultConnection"] = PostgresContainer.GetConnectionString(),
                ["ConnectionStrings:Redis"] = RedisContainer.GetConnectionString(),
                ["Logging:LogLevel:Default"] = "Warning"
            });
        });

        builder.ConfigureServices(services =>
        {
            // Replace TimeProvider
            services.Remove(services.Single(d => d.ServiceType == typeof(TimeProvider)));
            services.AddSingleton<TimeProvider>(TimeProvider);
        });

        builder.UseEnvironment("Testing");
    }

    public new async Task DisposeAsync()
    {
        if (_postgresContainer != null) await _postgresContainer.DisposeAsync();
        if (_redisContainer != null) await _redisContainer.DisposeAsync();
        await base.DisposeAsync();
    }
}
```text

### Collection Fixture Pattern

```csharp
[CollectionDefinition("Integration Tests")]
public class IntegrationTestCollection : ICollectionFixture<TestWebApplicationFactory>
{
    public const string Name = "Integration Tests";
}
```text

### Test Base Class

```csharp
[Collection("Integration Tests")]
public abstract class IntegrationTestBase : IAsyncLifetime
{
    protected readonly TestWebApplicationFactory Factory;
    protected readonly HttpClient HttpClient;
    protected readonly DatabaseManager DatabaseManager;
    protected readonly IFlurlClient FlurlClient;

    protected IntegrationTestBase(TestWebApplicationFactory factory)
    {
        Factory = factory;
        HttpClient = factory.CreateClient();
        DatabaseManager = new DatabaseManager(factory.PostgresContainer.GetConnectionString());
        FlurlClient = new FlurlClient(HttpClient);
    }

    public virtual async Task InitializeAsync()
    {
        await DatabaseManager.InitializeDatabaseAsync();
    }

    public virtual async Task DisposeAsync()
    {
        await DatabaseManager.CleanDatabaseAsync();
        FlurlClient.Dispose();
    }

    protected void ResetTime()
    {
        Factory.TimeProvider.SetUtcNow(new DateTimeOffset(2024, 1, 1, 0, 0, 0, TimeSpan.Zero));
    }

    protected void AdvanceTime(TimeSpan timeSpan)
    {
        Factory.TimeProvider.Advance(timeSpan);
    }
}
```text

## Flurl Simplifies URL Construction

Flurl provides fluent API for building complex URLs:

```csharp
// Traditional approach
var url = $"/products?pageSize={pageSize}&page={page}&keyword={keyword}";

// Using Flurl
var url = "/products"
    .SetQueryParam("pageSize", 5)
    .SetQueryParam("page", 2)
    .SetQueryParam("keyword", "special");
```text

## Testing Examples

### Successful Product Creation Test

```csharp
[Fact]
public async Task CreateProduct_WithValidData_ShouldCreateProductSuccessfully()
{
    // Arrange
    var request = new ProductCreateRequest { Name = "New Product", Price = 299.99m };

    // Act
    var response = await HttpClient.PostAsJsonAsync("/products", request);

    // Assert
    response.Should().Be201Created()
        .And.Satisfy<ProductResponse>(product =>
        {
            product.Id.Should().NotBeEmpty();
            product.Name.Should().Be("New Product");
            product.Price.Should().Be(299.99m);
        });
}
```text

### Validation Error Test

```csharp
[Fact]
public async Task CreateProduct_WhenProductNameIsEmpty_ShouldReturn400BadRequest()
{
    // Arrange
    var invalidRequest = new ProductCreateRequest { Name = "", Price = 100.00m };

    // Act
    var response = await HttpClient.PostAsJsonAsync("/products", invalidRequest);

    // Assert
    response.Should().Be400BadRequest()
        .And.Satisfy<ValidationProblemDetails>(problem =>
        {
            problem.Type.Should().Be("https://tools.ietf.org/html/rfc9110#section-15.5.1");
            problem.Title.Should().Be("One or more validation errors occurred.");
            problem.Errors.Should().ContainKey("Name");
            problem.Errors["Name"].Should().Contain("Product name cannot be empty");
        });
}
```text

### Resource Not Found Test

```csharp
[Fact]
public async Task GetById_WhenProductDoesNotExist_ShouldReturn404WithProblemDetails()
{
    // Arrange
    var nonExistentId = Guid.NewGuid();

    // Act
    var response = await HttpClient.GetAsync($"/Products/{nonExistentId}");

    // Assert
    response.Should().Be404NotFound()
        .And.Satisfy<ProblemDetails>(problem =>
        {
            problem.Type.Should().Be("https://httpstatuses.com/404");
            problem.Title.Should().Be("Product does not exist");
            problem.Status.Should().Be(404);
        });
}
```text

### Pagination Query Test

```csharp
[Fact]
public async Task GetProducts_WithPaginationParameters_ShouldReturnCorrectPagedResult()
{
    // Arrange
    await TestHelpers.SeedProductsAsync(DatabaseManager, 15);

    // Act - Use Flurl to construct QueryString
    var url = "/products"
        .SetQueryParam("pageSize", 5)
        .SetQueryParam("page", 2);

    var response = await HttpClient.GetAsync(url);

    // Assert
    response.Should().Be200Ok()
        .And.Satisfy<PagedResult<ProductResponse>>(result =>
        {
            result.Total.Should().Be(15);
            result.PageSize.Should().Be(5);
            result.Page.Should().Be(2);
            result.Items.Should().HaveCount(5);
        });
}
```text

## Data Management Strategy

### TestHelpers Design

```csharp
public static class TestHelpers
{
    public static ProductCreateRequest CreateProductRequest(
        string name = "Test Product",
        decimal price = 100.00m)
    {
        return new ProductCreateRequest { Name = name, Price = price };
    }

    public static async Task SeedProductsAsync(DatabaseManager dbManager, int count)
    {
        var tasks = Enumerable.Range(1, count)
            .Select(i => SeedSpecificProductAsync(dbManager, $"Product {i:D2}", i * 10.0m));
        await Task.WhenAll(tasks);
    }
}
```text

### SQL Script Externalization

```text
tests/Integration/
└── SqlScripts/
    └── Tables/
        └── CreateProductsTable.sql
```text

## Best Practices

### 1. Test Structure Design

- **Single Responsibility**: Each test focuses on one specific scenario
- **3A Pattern**: Clear separation of Arrange, Act, Assert
- **Clear Naming**: Method name expresses test intent

### 2. Error Handling Validation

- **ValidationProblemDetails**: Validate error response format
- **ProblemDetails**: Validate business exception response
- **HTTP Status Code**: Confirm correct status code

### 3. Performance Considerations

- **Container Sharing**: Use Collection Fixture
- **Data Cleanup**: Clean data after tests, don't recreate containers
- **Parallel Execution**: Ensure test independence

## Dependency Packages

```xml
<PackageReference Include="xunit" Version="2.9.3" />
<PackageReference Include="AwesomeAssertions" Version="9.1.0" />
<PackageReference Include="Testcontainers.PostgreSql" Version="4.0.0" />
<PackageReference Include="Testcontainers.Redis" Version="4.0.0" />
<PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="9.0.0" />
<PackageReference Include="Flurl" Version="4.0.0" />
<PackageReference Include="Respawn" Version="6.2.1" />
```text

## Project Structure

```text
src/
├── Api/                          # Web API layer
├── Application/                  # Application service layer
├── Domain/                       # Domain model
└── Infrastructure/               # Infrastructure layer
tests/
└── Integration/
    ├── Fixtures/
    │   ├── TestWebApplicationFactory.cs
    │   ├── IntegrationTestCollection.cs
    │   └── IntegrationTestBase.cs
    ├── Handlers/
    │   ├── GlobalExceptionHandler.cs
    │   └── FluentValidationExceptionHandler.cs
    ├── Helpers/
    │   ├── DatabaseManager.cs
    │   └── TestHelpers.cs
    ├── SqlScripts/
    │   └── Tables/
    └── Controllers/
        └── ProductsControllerTests.cs
```text

## Reference Resources

### Original Articles

This skill content is distilled from the "Old School Software Engineer's Testing Practice - 30 Day Challenge" article series:

- **Day 23 - Integration Testing in Practice: Web API Service Integration Testing**
  - Article: https://ithelp.ithome.com.tw/articles/10376873
  - Sample code: https://github.com/kevintsengtw/30Days_in_Testing_Samples/tree/main/day23

### Official Documentation

- [ASP.NET Core Integration Testing](https://docs.microsoft.com/aspnet/core/test/integration-tests)
- [IExceptionHandler Documentation](https://learn.microsoft.com/aspnet/core/fundamentals/error-handling)
- [ProblemDetails RFC 7807](https://tools.ietf.org/html/rfc7807)
- [Testcontainers for .NET](https://dotnet.testcontainers.org/)
- [AwesomeAssertions](https://awesomeassertions.org/)
- [Flurl HTTP Client](https://flurl.dev/)
- [Respawn](https://github.com/jbogard/Respawn)