Migrate from xUnit to TUnit and adopt Microsoft.Testing.Platform v2

[BenjaminMichaelis]

Why

TUnit is the modern test framework for .NET with source generation, AOT compatibility, and native support for Microsoft.Testing.Platform v2 (MTP v2). This migration:

• Standardizes on MTP v2 for the .NET 10 SDK (native dotnet test support, no VSTest bridge required)
• Removes legacy dependencies (xUnit, coverlet) in favor of TUnit's built-in MTP coverage support
• Follows the official TUnit xUnit migration guide for maximum compatibility and maintainability

Approach

Framework Conversion (using TUnit fixer):

• Installed TUnit alongside xUnit temporarily, disabled implicit usings, ran dotnet format analyzers --severity info --diagnostics TUXU0001 to auto-convert attributes
• [Fact] → [Test], [Theory] → [Test], [InlineData] → [Arguments], [MemberData] → [MethodDataSource]

Custom xUnit Infrastructure Replacement:

• Deleted ConditionalFactAttribute.cs and ConditionalFactDiscoverer.cs (xUnit SDK dependent, not portable)
• Created OperatingSystemSkipAttributes.cs with WindowsOnlyAttribute and NonWindowsOnlyAttribute using TUnit's native SkipAttribute pattern
• Updated 4 platform-specific tests to use new skip attributes

Data Source Fix:

• Fixed TrxParserRegressionTests.cs data source: converted TheoryData<> initialization to tuple-based IEnumerable<(string, string[])> (valid syntax for TUnit method data sources)

Project Runtime Changes:

• Added <OutputType>Exe</OutputType> (required for MTP v2 executables)
• Removed xunit, xunit.runner.visualstudio, Microsoft.NET.Test.Sdk, coverlet.collector
• Updated global.json with MTP v2 runner opt-in for .NET 10
• Updated CI test command to use MTP-native coverage flags (--coverage --coverage-output-format cobertura)

Result

• All 63 tests passing; 2 platform-specific tests correctly skipped on Windows
• MTP coverage command produces Cobertura XML output as expected
• No breaking changes to test behavior, only framework/runner swap

Testing

• Baseline: 61 xUnit tests passing
• Post-migration: 63 TUnit tests passing (added 2 by fixing method data source return type), 2 platform-conditional skips working as intended
• Coverage flags validated and producing output

[Copilot]

Pull request overview

Migrates the test project from xUnit/VSTest to TUnit on Microsoft.Testing.Platform v2 (MTP v2), updating test attributes/data sources and aligning CI to run tests with MTP-native coverage flags.

Changes:

• Convert xUnit [Fact]/[Theory]/[InlineData]/[MemberData] usage to TUnit [Test]/[Arguments]/[MethodDataSource].
• Replace custom xUnit ConditionalFact infrastructure with TUnit-native OS skip attributes (WindowsOnly / NonWindowsOnly).
• Update test project/CI to use MTP v2 (test project OutputType=Exe, global.json runner opt-in, CI coverage flags) and remove legacy test/coverage packages.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
TrxLib.Tests/TrxParserTests.cs	Converts tests to TUnit attributes and swaps platform-conditional tests to WindowsOnly/NonWindowsOnly.
TrxLib.Tests/TrxParserRegressionTests.cs	Updates theory/data-source pattern to TUnit arguments + method data source friendly shape.
TrxLib.Tests/TestResultTests.cs	Converts parameterized and non-parameterized tests to TUnit attributes.
TrxLib.Tests/OperatingSystemSkipAttributes.cs	Adds TUnit SkipAttribute-based OS gating attributes to replace xUnit ConditionalFact.
TrxLib.Tests/TrxLib.Tests.csproj	Switches package references from xUnit/VSTest/coverlet to TUnit and sets OutputType=Exe for MTP v2.
TrxLib.Tests/ConditionalFactDiscoverer.cs	Removes xUnit conditional discovery infrastructure.
TrxLib.Tests/ConditionalFactAttribute.cs	Removes xUnit ConditionalFact attribute.
global.json	Opts into Microsoft.Testing.Platform runner selection for dotnet test.
Directory.Packages.props	Drops xUnit/VSTest/coverlet versions and adds TUnit version.
.github/workflows/build-and-test.yml	Updates CI dotnet test invocation to include MTP-native coverage flags.

Comments suppressed due to low confidence (1)

TrxLib.Tests/TrxParserTests.cs:142

• IsWindows() / NotWindows() helpers (and the using System.Runtime.InteropServices; at the top of the file) are now unused after switching to [WindowsOnly] / [NonWindowsOnly]. Please remove the dead methods/usings to keep the test file tidy and avoid unused-member warnings from analyzers.

    [Test, WindowsOnly]
    public void Parse_Example1WindowsTrx_ParsesTestProjectDirectoryCorrectly()
    {
        var results = TrxParser.Parse(new FileInfo(GetSampleFilePath(Path.Combine("example1_Windows.trx"))));
        var test = results.Single(r => r.FullyQualifiedTestName == "Microsoft.DotNet.Cli.Sln.Internal.Tests.GivenAnSlnFile.WhenGivenAValidPathItReadsModifiesThenWritesAnSln");

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