C# Unit Testing with XUnit v3

Follow these practices when writing, reviewing, or refactoring C# unit tests. The stack is XUnit v3 + NSubstitute + FluentAssertions + Atc.Test. Do not use Moq.

When adding tests to an existing project, read nearby test files first and match the established style before applying these defaults.

Naming and Structure

Name tests ClassName_MethodUnderTest_ExpectedBehavior so the test runner output reads like a specification.

Every test follows the Arrange-Act-Assert pattern with explicit comments:

[Fact]
public void OrderService_CalculateTotal_ReturnsZero_WhenCartIsEmpty()
{
    // Arrange
    var sut = new OrderService();
    var emptyCart = new Cart();