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

Unit Test Structure

Arrange-Act-Assert

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

[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

[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

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

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

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

Running Tests

# 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/