Writing a unit test in JUnit 5 and xUnit.net: annotations, lifecycle and parameterised cases side by side

Goal

Write a test that a reader from the other ecosystem recognises at once: one behaviour per test, setup visible, repeated cases expressed as data.

Prerequisites

JUnit Jupiter (junit-jupiter on the test classpath, junit-jupiter-params for parameterised tests) or xUnit.net (xunit plus a runner) in a project whose build already runs tests; familiarity with the general unit-test structure article on this wiki.

Steps

1. Name the class after the unit and the method after the behaviour: OrderTotalTest.appliesDiscountAboveThreshold() in Java, OrderTotalTests.AppliesDiscountAboveThreshold() in C#.
2. Mark the test: JUnit @Test from org.junit.jupiter.api; xUnit [Fact]. Both frameworks instantiate the test class afresh for each test method by default: JUnit's documentation describes the per-method lifecycle and the @TestInstance(Lifecycle.PER_CLASS) switch, and xUnit's shared-context page states that the constructor runs for every single test.
3. Put per-test setup where the framework expects it: JUnit @BeforeEach and @AfterEach methods; xUnit the constructor and Dispose(), or IAsyncLifetime when setup is asynchronous. Keep state in instance fields, not statics.
4. Share expensive context deliberately: JUnit @BeforeAll on a static method (non-static under the per-class lifecycle); xUnit IClassFixture<T> for one fixture instance per test class and ICollectionFixture<T> across classes, both created before the first test and disposed after the last.
5. Turn copy-pasted tests into data: JUnit @ParameterizedTest with @ValueSource, @CsvSource or @MethodSource (the documentation requires at least one source); xUnit [Theory] with [InlineData(...)], [MemberData] or [ClassData].
6. Assert one behaviour: JUnit assertEquals(expected, actual), assertThrows(Type.class, () -> ...), assertAll(...) to group related assertions; xUnit Assert.Equal(expected, actual), Assert.Throws<T>(() => ...). Both put the expected value first.
7. Run one test from the command line to prove the wiring: ./gradlew test --tests OrderTotalTest or ./mvnw -Dtest=OrderTotalTest test; dotnet test --filter FullyQualifiedName~OrderTotalTests.

Expected result

Each test file reads the same way in both languages: construction, action, assertion; parameter tables replace duplicated methods; setup cost is visible in the lifecycle hook that carries it.

Limits and test basis

Assertion libraries (AssertJ, Hamcrest, FluentAssertions, Shouldly) change the syntax of step 6, not the structure. Parallel execution differs between the frameworks and must be checked in their documentation before tests share static state. The procedure follows the cited documentation; no comparison of speed or defect rates is claimed.