csharp-testing

Use xUnit's [Fact] attribute for single tests and follow the Arrange-Act-Assert structure with FluentAssertions for readable assertions. Create a fresh system-under-test instance in the constructor, since xUnit creates a new class instance per test.

NSubstitute offers a concise syntax using Substitute.For<T>() and Returns() for stubbing, while Moq uses Mock<T> with Setup() calls. Both verify calls on dependencies; NSubstitute's Received() API reads more naturally for verification.

Use WebApplicationFactory<Program> with IClassFixture to boot the API in-memory and override services like DbContext via ConfigureServices. For real database testing, use Testcontainers to spin up PostgreSQL in Docker and run migrations before tests.

Yes, xUnit supports parameterized tests via [Theory] with [InlineData] for simple values and [MemberData] for complex object cases. This avoids duplicating test methods for multiple input-output combinations.

Flakiness often comes from using Thread.Sleep instead of Task.Delay with timeouts, shared mutable state between tests, or asserting on implementation details. Use fresh instances per test and polling helpers for async conditions.