Convert Mono.Linker.Tests from MSTest to xUnit

[Copilot]

Description

Converts Mono.Linker.Tests — the last remaining MSTest project in illink — to xUnit, aligning it with the rest of dotnet/runtime's test infrastructure.

Changes

Project setup

• Remove <TestRunnerName>MSTest</TestRunnerName> from csproj; xUnit is now the default via Arcade
• Add explicit Microsoft.DotNet.XUnitExtensions PackageReference (not auto-imported for src/tools/illink/)

Test data / dynamic data

• TestDatabase: return types changed from IEnumerable<TestDataRow<TestCase>> → IEnumerable<object[]>; MSTestCasesBySuiteName → TestCasesBySuiteName
• MemberAssertionsCollector.GetMemberAssertionsData: returns IEnumerable<object[]> instead of IEnumerable<TestDataRow<T>>
• TestCase.ToString() added to emit DisplayName for readable xUnit test names
• [DynamicData(..., new object[] { typeof(T) })] with parameterized data source → parameterless [MemberData] method per test class

Test attributes

• [TestClass] removed; [TestMethod] → [Fact]; [DataRow] → [InlineData]
• [DynamicData] → [MemberData]; suite tests use [ConditionalTheory] (not plain [Theory]) to support dynamic skipping
• [TestInitialize] → constructor; [DoNotParallelize] removed (MSTest-only concept)
• AssemblyInfo.cs parallel configuration removed

Dynamic skipping
MSTest's Assert.Inconclusive doesn't translate directly to xUnit's skip mechanism. Most runtime skipping uses ConditionalTheory + SkipTestException from Microsoft.DotNet.XUnitExtensions. For CodegenAnnotationTests, declarative [SkipOnTargetFramework] attributes are used instead:

// Before (MSTest)
if (Environment.OSVersion.Platform == PlatformID.Win32NT)
    Assert.Inconclusive("These tests are not valid when trimming .NET Framework");
#if NET
Assert.Inconclusive("These tests are not valid when trimming .NET Core");
#endif

// After (xUnit)
[SkipOnTargetFramework(TargetFrameworkMonikers.Netcoreapp, "These tests are not valid when trimming .NET Core")]
[SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework, "These tests are not valid when trimming .NET Framework")]

The [SkipOnTargetFramework] attribute adds a Category=Failing trait via SkipOnTargetFrameworkDiscoverer, which is automatically filtered out by the CI infrastructure via _withoutCategories=failing in eng/testing/tests.targets.

Assertions (all files)

MSTest	xUnit
Assert.AreEqual(a, b)	Assert.Equal(a, b)
Assert.IsTrue/IsFalse	Assert.True/False
Assert.IsNull/IsNotNull	Assert.Null/NotNull
Assert.IsEmpty	Assert.Empty
Assert.Inconclusive	throw new SkipTestException
CollectionAssert.AreEquivalent	Assert.Equal(a.OrderBy(...), b.OrderBy(...))
UnitTestAssertException	XunitException

Testing

Build: dotnet build src/tools/illink/test/Mono.Linker.Tests/Mono.Linker.Tests.csproj -c Release

Run: dotnet test src/tools/illink/test/Mono.Linker.Tests/Mono.Linker.Tests.csproj -c Release

Original prompt

Chronological Review: 1. User requested converting illink tests from MSTest to xUnit 2. Prior work (summarized): Explored repo, found only Mono.Linker.Tests uses MSTest, ran baseline build, delegated bulk conversion to sub-agent (22 files), committed as 9425ffe083f 3. Prior work: Discovered 31 test failures due to dynamic skip mechanism, fixed with ConditionalTheory + SkipTestException (3 files) 4. Prior work: Started code review skill, gathered diff context 5. Current session: Completed code review and amended commit

Recent Commands Analysis:

• Checked AreEqual→Assert.True conversions in ResultChecker.cs diff
• Verified SkipTestException and ConditionalTheory usage in TestSuites.cs
• Found only 1 instance of Assert.True(a == b, msg) pattern at line 259
• Wrote code review (LGTM with one 💡 suggestion about Assert.NotNull message loss)
• Staged 3 uncommitted files and amended commit
• Final commit: 2a2d31b5610
• Called task_complete

Progress: Everything is DONE. The task is complete.

1. Conversation Overview: - Primary Objectives: "Update the illink tests from MSTest to xunit." - Session Context: User requested converting `Mono.Linker.Tests` (the last MSTest test project in illink) to xUnit. Work progressed through exploration → bulk conversion → fixing dynamic skip failures → code review → final commit. - User Intent Evolution: Single clear request, no changes in direction.

2. Technical Foundation:

   • xUnit v2 runner (2.9.x) with xUnit v3 assertions (Microsoft.DotNet.XUnitAssert 3.2.2-beta) — the dotnet/runtime standard setup
   • Arcade SDK: Tests.targets defaults TestRunnerName=XUnit; removing <TestRunnerName>MSTest</TestRunnerName> switches to xUnit packages automatically
   • Microsoft.DotNet.XUnitExtensions: Provides ConditionalTheory/ConditionalFact and SkipTestException for dynamic test skipping. NOT auto-imported for src/tools/illink/ (only for src/libraries/ and src/tests/), so needed explicit PackageReference.
   • SDK version: 11.0.100-preview.3.26170.106

3. Codebase Status:

   • All 22 files committed in single commit 2a2d31b5610 on branch illink-xunit
   • Key files:
     • Mono.Linker.Tests.csproj — Removed TestRunnerName=MSTest, added Microsoft.DotNet.XUnitExtensions PackageReference
     • TestSuites.cs — 47 [ConditionalTheory] methods, [MemberData] instead of [DynamicData], SkipTestException for dynamic skips
     • TestRunner.cs — IgnoreTest throws SkipTestException, catches XunitException instead of UnitTestAssertException
     • TestDatabase.cs — object[] instead of TestDataRow<TestCase>, renamed MSTestCasesBySuiteName → TestCasesBySuiteName
     • TestCase.cs — Added ToString() override returning DisplayName
     • ResultChecker.cs — Dozens of assertion conversions; 3 Assert.NotNull calls lost custom messages; 1 Assert.True(a == b, msg) pattern at line 259
     • 16 other files with attribute/using/assertion conversions

4. Problem Resolution:

   • 31 test failures from dynamic skip: SkipException.ForSkip() (Xunit.Sdk) produces $XunitDynamicSkip$ prefix not caught by xUnit v2 runner. Fixed by using ConditionalTheory + SkipTestException from Microsoft.DotNet.XUnitExtensions.
   • Assert.NotNull message loss: xUnit's Assert.NotNull doesn't accept messages. Accepted as known tradeoff (3 instances).
   • Assert.AreEqual with message: One instance converted to Assert.True(a == b, msg) to preserve diagnostic output including linker output.

5. Progress Tracking:

   • ✅ Baseline build succeeded
   • ✅ All 22 files converted from MSTest to xUnit
   • ✅ Dynamic skip mechanism fixed (ConditionalTheory + SkipTestException)
   • ✅ Build passes (0 errors, 0 warnings)
   • ✅ Tests pass: 0 Failed, 1125 Passed, 31 Skipped
   • ✅ Code review completed (LGTM, one 💡 suggestion)
   • ✅ All changes committed in single amended commit 2a2d31b5610
   • ✅ task_complete called

6. Active Work State:

   • Task is COMPLETE. task_complete was called with full summary.

7. Recent Operations:

   • Last Agent Commands:
     1. bash: git diff HEAD~1 -- ResultChecker.cs | grep "Assert.AreEqual.*Assert.True" — no results
     2. bash: git diff HEAD~1 -- ResultChecker.cs | grep -A1 "Assert.AreEqual" — found the 3 AreEqual conversions including the Assert.True pattern
     3. bash: grep "Assert.True.*==\|Assert.False.*!=" ResultChecker.cs — confirmed only 1 instance at line 259
     4. bash: grep "SkipTestException" TestSuites.cs — confirmed 2 usages
     5. Wrote code review output (LGTM with findings)
     6. bash: git add -A && git diff --cached --stat — staged 3 files (csproj, TestSuites.cs, TestRunner.cs)
     7. bash: git commit --amend — amended commit to 2a2d31b5610 with 22 files, 369 insertions, 396 deletions
     8. bash: git log --oneline -2 — verified commit on illink-xunit branch
     9. `ta...

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