csharp-testing

使い方

インストールコマンドをコピーし、自分の環境へ導入する前にソースを確認します。
必要なツール、認証情報、対応環境を原文で確認します。
入力と成果物を指定し、出力を資料と照合します。

npx skills add https://github.com/affaan-m/ecc --skill csharp-testing

開始前の確認

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スター数や導入数は活動の指標であり、独立した品質・安全性試験ではありません。

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C# Testing Patterns

Comprehensive testing patterns for .NET applications using xUnit, FluentAssertions, and modern testing practices.

When to Activate

Writing new tests for C# code
Reviewing test quality and coverage
Setting up test infrastructure for .NET projects
Debugging flaky or slow tests

Test Framework Stack

Tool	Purpose
xUnit	Test framework (preferred for .NET)
FluentAssertions	Readable assertion syntax
NSubstitute or Moq	Mocking dependencies
Testcontainers	Real infrastructure in integration tests
WebApplicationFactory	ASP.NET Core integration tests
Bogus	Realistic test data generation

Unit Test Structure

Arrange-Act-Assert

csharp

public sealed class OrderServiceTests
{
    private readonly IOrderRepository _repository = Substitute.For<IOrderRepository>();
    private readonly ILogger<OrderService> _logger = Substitute.For<ILogger<OrderService>>();
    private readonly OrderService _sut;

    public OrderServiceTests()
    {
        _sut = new OrderService(_repository, _logger);
    }

    [Fact]
    public async Task PlaceOrderAsync_ReturnsSuccess_WhenRequestIsValid()
    {
        // Arrange
        var request = new CreateOrderRequest
        {
            CustomerId = "cust-123",
            Items = [new OrderItem("SKU-001", 2, 29.99m)]
        };

        // Act
        var result = await _sut.PlaceOrderAsync(request, CancellationToken.None);

        // Assert
        result.IsSuccess.Should().BeTrue();
        result.Value.Should().NotBeNull();
        result.Value!.CustomerId.Should().Be("cust-123");
    }

    [Fact]
    public async Task PlaceOrderAsync_ReturnsFailure_WhenNoItems()
    {
        // Arrange
        var request = new CreateOrderRequest
        {
            CustomerId = "cust-123",
            Items = []
        };

        // Act
        var result = await _sut.PlaceOrderAsync(request, CancellationToken.None);

        // Assert
        result.IsSuccess.Should().BeFalse();
        result.Error.Should().Contain("at least one item");
    }
}

Parameterized Tests with Theory

csharp

[Theory]
[InlineData("", false)]
[InlineData("a", false)]
[InlineData("ab@c.d", false)]
[InlineData("user@example.com", true)]
[InlineData("user+tag@example.co.uk", true)]
public void IsValidEmail_ReturnsExpected(string email, bool expected)
{
    EmailValidator.IsValid(email).Should().Be(expected);
}

[Theory]
[MemberData(nameof(InvalidOrderCases))]
public async Task PlaceOrderAsync_RejectsInvalidOrders(CreateOrderRequest request, string expectedError)
{
    var result = await _sut.PlaceOrderAsync(request, CancellationToken.None);

    result.IsSuccess.Should().BeFalse();
    result.Error.Should().Contain(expectedError);
}

public static TheoryData<CreateOrderRequest, string> InvalidOrderCases => new()
{
    { new() { CustomerId = "", Items = [ValidItem()] }, "CustomerId" },
    { new() { CustomerId = "c1", Items = [] }, "at least one item" },
    { new() { CustomerId = "c1", Items = [new("", 1, 10m)] }, "SKU" },
};

Mocking with NSubstitute

csharp

[Fact]
public async Task GetOrderAsync_ReturnsNull_WhenNotFound()
{
    // Arrange
    var orderId = Guid.NewGuid();
    _repository.FindByIdAsync(orderId, Arg.Any<CancellationToken>())
        .Returns((Order?)null);

    // Act
    var result = await _sut.GetOrderAsync(orderId, CancellationToken.None);

    // Assert
    result.Should().BeNull();
}

[Fact]
public async Task PlaceOrderAsync_PersistsOrder()
{
    // Arrange
    var request = ValidOrderRequest();

    // Act
    await _sut.PlaceOrderAsync(request, CancellationToken.None);

    // Assert — verify the repository was called
    await _repository.Received(1).AddAsync(
        Arg.Is<Order>(o => o.CustomerId == request.CustomerId),
        Arg.Any<CancellationToken>());
}

ASP.NET Core Integration Tests

WebApplicationFactory Setup

csharp

public sealed class OrderApiTests : IClassFixture<WebApplicationFactory<Program>>
{
    private readonly HttpClient _client;

    public OrderApiTests(WebApplicationFactory<Program> factory)
    {
        _client = factory.WithWebHostBuilder(builder =>
        {
            builder.ConfigureServices(services =>
            {
                // Replace real DB with in-memory for tests
                services.RemoveAll<DbContextOptions<AppDbContext>>();
                services.AddDbContext<AppDbContext>(options =>
                    options.UseInMemoryDatabase("TestDb"));
            });
        }).CreateClient();
    }

    [Fact]
    public async Task GetOrder_Returns404_WhenNotFound()
    {
        var response = await _client.GetAsync($"/api/orders/{Guid.NewGuid()}");

        response.StatusCode.Should().Be(HttpStatusCode.NotFound);
    }

    [Fact]
    public async Task CreateOrder_Returns201_WithValidRequest()
    {
        var request = new CreateOrderRequest
        {
            CustomerId = "cust-1",
            Items = [new("SKU-001", 1, 19.99m)]
        };

        var response = await _client.PostAsJsonAsync("/api/orders", request);

        response.StatusCode.Should().Be(HttpStatusCode.Created);
        response.Headers.Location.Should().NotBeNull();
    }
}

Testing with Testcontainers

csharp

public sealed class PostgresOrderRepositoryTests : IAsyncLifetime
{
    private readonly PostgreSqlContainer _postgres = new PostgreSqlBuilder()
        .WithImage("postgres:16-alpine")
        .Build();

    private AppDbContext _db = null!;

    public async Task InitializeAsync()
    {
        await _postgres.StartAsync();
        var options = new DbContextOptionsBuilder<AppDbContext>()
            .UseNpgsql(_postgres.GetConnectionString())
            .Options;
        _db = new AppDbContext(options);
        await _db.Database.MigrateAsync();
    }

    public async Task DisposeAsync()
    {
        await _db.DisposeAsync();
        await _postgres.DisposeAsync();
    }

    [Fact]
    public async Task AddAsync_PersistsOrder()
    {
        var repo = new SqlOrderRepository(_db);
        var order = Order.Create("cust-1", [new OrderItem("SKU-001", 2, 10m)]);

        await repo.AddAsync(order, CancellationToken.None);

        var found = await repo.FindByIdAsync(order.Id, CancellationToken.None);
        found.Should().NotBeNull();
        found!.Items.Should().HaveCount(1);
    }
}

Test Organization

tests/
  MyApp.UnitTests/
    Services/
      OrderServiceTests.cs
      PaymentServiceTests.cs
    Validators/
      EmailValidatorTests.cs
  MyApp.IntegrationTests/
    Api/
      OrderApiTests.cs
    Repositories/
      OrderRepositoryTests.cs
  MyApp.TestHelpers/
    Builders/
      OrderBuilder.cs
    Fixtures/
      DatabaseFixture.cs

Test Data Builders

csharp

public sealed class OrderBuilder
{
    private string _customerId = "cust-default";
    private readonly List<OrderItem> _items = [new("SKU-001", 1, 10m)];

    public OrderBuilder WithCustomer(string customerId)
    {
        _customerId = customerId;
        return this;
    }

    public OrderBuilder WithItem(string sku, int quantity, decimal price)
    {
        _items.Add(new OrderItem(sku, quantity, price));
        return this;
    }

    public Order Build() => Order.Create(_customerId, _items);
}

// Usage in tests
var order = new OrderBuilder()
    .WithCustomer("cust-vip")
    .WithItem("SKU-PREMIUM", 3, 99.99m)
    .Build();

Common Anti-Patterns

Anti-Pattern	Fix
Testing implementation details	Test behavior and outcomes
Shared mutable test state	Fresh instance per test (xUnit does this via constructors)
`Thread.Sleep` in async tests	Use `Task.Delay` with timeout, or polling helpers
Asserting on `ToString()` output	Assert on typed properties
One giant assertion per test	One logical assertion per test
Test names describing implementation	Name by behavior: `Method_ExpectedResult_WhenCondition`
Ignoring `CancellationToken`	Always pass and verify cancellation

Running Tests

bash

# Run all tests
dotnet test

# Run with coverage
dotnet test --collect:"XPlat Code Coverage"

# Run specific project
dotnet test tests/MyApp.UnitTests/

# Filter by test name
dotnet test --filter "FullyQualifiedName~OrderService"

# Watch mode during development
dotnet watch test --project tests/MyApp.UnitTests/

使い方

開始前の確認

C# Testing Patterns

When to Activate

Test Framework Stack

Unit Test Structure

Arrange-Act-Assert

Parameterized Tests with Theory

Mocking with NSubstitute

ASP.NET Core Integration Tests

WebApplicationFactory Setup

Testing with Testcontainers

Test Organization

Test Data Builders

Common Anti-Patterns

Running Tests

タスクに関連する項目

関連する Skills

scientific-thinking-literature-review

docker-patterns

agentic-engineering

backend-patterns

csharp-testing

使い方

開始前の確認

C# Testing Patterns

When to Activate

Test Framework Stack

Unit Test Structure

Arrange-Act-Assert

Parameterized Tests with Theory

Mocking with NSubstitute

ASP.NET Core Integration Tests

WebApplicationFactory Setup

Testing with Testcontainers

Test Organization

Test Data Builders

Common Anti-Patterns

Running Tests

タスクに関連する項目

laravel-tdd

coding-standards

springboot-security

django-security

関連する Skills

scientific-thinking-literature-review

docker-patterns

agentic-engineering

backend-patterns