csharp-testing

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C# Testing Patterns

C#测试模式

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

面向.NET应用的全面测试模式，采用xUnit、FluentAssertions及现代测试实践。

When to Activate

适用场景

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

为C#代码编写新测试时
审查测试质量与覆盖率时
为.NET项目搭建测试基础设施时
调试不稳定或运行缓慢的测试时

Test Framework Stack

测试框架栈

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

工具	用途
xUnit	测试框架（.NET首选）
FluentAssertions	易读的断言语法
NSubstitute 或 Moq	Mock依赖项
Testcontainers	集成测试中的真实基础设施
WebApplicationFactory	ASP.NET Core集成测试
Bogus	真实测试数据生成

Unit Test Structure

单元测试结构

Arrange-Act-Assert

准备-执行-断言（AAA模式）

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");
    }
}

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

使用Theory实现参数化测试

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" },
};

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

使用NSubstitute进行Mock

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>());
}

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

ASP.NET Core集成测试

WebApplicationFactory Setup

WebApplicationFactory配置

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();
    }
}

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

使用Testcontainers进行测试

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);
    }
}

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

测试组织结构

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

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

测试数据构建器

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();

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

常见反模式

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

反模式	解决方案
测试实现细节	测试行为与结果
共享可变测试状态	每个测试使用全新实例（xUnit已通过构造函数默认实现）
异步测试中使用 `Thread.Sleep`	使用带超时的 `Task.Delay` 或轮询助手工具
对 `ToString()` 输出做断言	对类型化属性做断言
单测试包含大量断言	每个测试仅做一个逻辑断言
测试名称描述实现细节	按行为命名： `方法名_预期结果_触发条件`
忽略 `CancellationToken`	始终传递并验证取消逻辑

Running Tests

运行测试

bash

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bash

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Run all tests

运行所有测试

dotnet test

Run with coverage

带覆盖率运行

dotnet test --collect:"XPlat Code Coverage"

Run specific project

运行指定项目的测试

dotnet test tests/MyApp.UnitTests/

Filter by test name

按测试名称过滤

dotnet test --filter "FullyQualifiedName~OrderService"

Watch mode during development

开发阶段监听模式

dotnet watch test --project tests/MyApp.UnitTests/

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dotnet watch test --project tests/MyApp.UnitTests/

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