xunit-tests

Overview

Per xunit.net:

xUnit.net is the current .NET test standard (used by .NET Foundation projects + Microsoft's own .NET runtime). v3 released 2024; v2 still widely used in production.

Step 1 - Install

dotnet new xunit -n MyProjectTests
# Or in existing project:
dotnet add package xunit
dotnet add package xunit.runner.visualstudio
dotnet add package Microsoft.NET.Test.Sdk

Step 2 - First test

using Xunit;

public class CalculatorTests
{
    [Fact]
    public void Adds_TwoNumbers()
    {
        Assert.Equal(3, Calculator.Add(1, 2));
    }
}

Run: dotnet test. Verify: assert the run reports Passed! with the expected test count before proceeding; if it exits non-zero or discovers 0 tests, confirm the class is public, the method carries [Fact], and all three packages (xunit, xunit.runner.visualstudio, Microsoft.NET.Test.Sdk) are installed, then re-run.

Step 3 - Parametrized tests

Per xn-docs:

[Theory]
[InlineData(1, 2, 3)]
[InlineData(0, 0, 0)]
[InlineData(-1, 1, 0)]
public void Adds_VariousInputs(int a, int b, int expected)
{
    Assert.Equal(expected, Calculator.Add(a, b));
}

// Class-based data source
public class AddTestData : IEnumerable<object[]>
{
    public IEnumerator<object[]> GetEnumerator()
    {
        yield return new object[] { 1, 2, 3 };
        yield return new object[] { 0, 0, 0 };
    }
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

[Theory]
[ClassData(typeof(AddTestData))]
public void Adds_FromClassData(int a, int b, int expected) { ... }

// Method-based
public static IEnumerable<object[]> AddCases =>
    new List<object[]> {
        new object[] { 1, 2, 3 },
        new object[] { 0, 0, 0 },
    };

[Theory]
[MemberData(nameof(AddCases))]
public void Adds_FromMemberData(int a, int b, int expected) { ... }

Step 4 - Skip + traits

[Fact(Skip = "Requires staging DB")]
public void SkippedTest() { }

[Fact]
[Trait("Category", "Integration")]
public void IntegrationTest() { }

// Filter:  dotnet test --filter "Category=Integration"

Fixtures + parallelism

For per-test, class (IClassFixture), and collection (ICollectionFixture) shared setup, plus assembly-level parallel config, see references/fixtures-and-parallelism.md.

Step 5 - Output (ITestOutputHelper)

xUnit suppresses Console.WriteLine in tests. Use ITestOutputHelper:

public class TestsWithOutput {
    private readonly ITestOutputHelper _output;
    public TestsWithOutput(ITestOutputHelper output) { _output = output; }

    [Fact]
    public void LogsContext() {
        _output.WriteLine("Test running at {0}", DateTime.UtcNow);
    }
}

Step 6 - Pair with FluentAssertions

result.Should().Be(42);
list.Should().HaveCount(3).And.Contain("alice");
result.Should().BeOfType<Success>().Which.Value.Should().Be(42);

See fluentassertions. Note: FluentAssertions changed license in 2024 (paid commercial; free for OSS); v6 is the last fully-free version.

Step 7 - CI integration

- run: dotnet test --logger "trx;LogFileName=test-results.trx" \
    --collect:"XPlat Code Coverage" -- DataCollectionRunSettings.DataCollectors.DataCollector.Configuration.Format=opencover
- uses: codecov/codecov-action@v4
  with: { files: ./coverage/coverage.opencover.xml }

Worked example

A service team writes xUnit coverage for a UserService backed by a database. They scaffold with dotnet new xunit (Step 1), cover pure logic like Calculator.Add with the [Theory] of Step 3, and park a staging-only case with the skip of Step 4. The DB-backed classes share one DatabaseFixture via a [Collection("DbCollection")] (see references/fixtures-and-parallelism.md) so they run sequentially against the shared connection while pure-logic tests parallelize. Diagnostics go through ITestOutputHelper (Step 5), and CI runs the coverage command of Step 7. Result: fast parallel unit coverage plus serialized DB-backed tests sharing one fixture.

Anti-patterns

Limitations

• xUnit's "constructor as setup, IDisposable as teardown" is unintuitive vs JUnit's annotations.
• Test discovery is slow on large solutions; use --filter.
• v2 vs v3 API has minor breaking changes; pin version per project.

References

• xn-docs - xUnit.net documentation
• xunit.net - landing
• learn.microsoft.com/en-us/dotnet/core/testing/unit-testing-with-dotnet-test - dotnet test
• references/fixtures-and-parallelism.md - fixtures + parallel execution
• nunit-tests, mstest-tests, fluentassertions - sister tools
• test-code-conventions