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 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 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

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/