xUnit -> MSTest Migration

Convert xUnit.net v2 or v3 tests to MSTest v4 without changing the target framework or test platform. A successful migration builds, discovers the same tests, and preserves pass/fail results and execution semantics.

Scope

Use this skill only when the project contains xUnit packages or source and the user wants MSTest. If the project already uses MSTest and contains no xUnit tests, report that no framework migration is needed and make no changes.

Do not combine this framework conversion with a target-framework upgrade or VSTest/MTP migration. Complete and verify one migration before starting another.

Response Mode

• Full migration request: inspect the project, make the edits, build, and run tests. Do not stop after giving a plan.
• Focused compile error or API question: inspect the relevant code and apply only that mapping. Do not narrate the entire workflow.
• Unsupported target framework: stop before changing packages. MSTest v4 requires .NET 8+ or .NET Framework 4.6.2+ for test applications; offer a separately approved TFM upgrade or MSTest v3 as the intermediate target.

For detailed mappings and examples, search references/mapping-cheatsheet.md for constructs actually present in the project and read only the matching sections. Do not load or reproduce the whole reference.

Fast Path

For a routine project migration, converge in four phases: one batched discovery read/search, one edit pass, one dotnet test, and one concise result. Do not:

• list a directory and then reread the same files through another tool
• try dotnet test --no-restore unless restore is already known to be current
• run separate restore, build, and test commands when dotnet test is sufficient
• rerun a passing test command or inspect unchanged files for confirmation

Use an existing CI/test result as the parity baseline when available. Run a new pre-edit baseline only when counts are unavailable and the migration contains data-driven tests, fixtures, skips, custom extensions, shared state, or other behavior whose parity cannot be established from source alone.

Workflow

1. Establish the baseline

1. In one discovery pass, batch-read the test projects plus Directory.Build.props, Directory.Packages.props, global.json, and runner configuration, and search the source for the high-risk constructs below.
2. State the detected source version:
   • xunit 2.x and related packages -> xUnit v2
   • xunit.v3 or xunit.v3.* -> xUnit v3
3. Identify VSTest or MTP from the project and repository configuration. Use platform-detection only when the platform is ambiguous, and preserve the detected platform.
4. Record the target frameworks and stop if MSTest v4 does not support them.
5. If the Fast Path requires a new baseline, run the existing test command once and record discovered, passed, failed, and skipped counts.
6. Inventory high-risk constructs before editing:
   • IClassFixture, ICollectionFixture, CollectionDefinition, custom FactAttribute/TheoryAttribute/DataAttribute
   • Assert.Throws, ThrowsAny, IsType, Record.Exception, event assertions
   • ITestOutputHelper, TestContext.Current, IAsyncLifetime
   • CollectionBehavior, xunit.runner.json, shared static or external state

2. Replace packages without switching runners

Remove xUnit packages from project files and central package files. This includes xunit*, xunit.v3.*, xunit.runner.visualstudio, YTest.MTP.XUnit2, and xUnit-specific companion packages that are being replaced.

Default to the MSTest v4 metapackage for an incremental conversion:

<PackageReference Include="MSTest" Version="4.1.0" />

This keeps VSTest available through the metapackage's compatible Microsoft.NET.Test.Sdk dependency. Remove a stale explicit Microsoft.NET.Test.Sdk reference or update it to the minimum required by the chosen MSTest version (MSTest 4.1.0 requires 18.0.1+); otherwise restore fails with NU1605. Use MSTest.Sdk only when the project already uses it elsewhere or the user explicitly requests it. MSTest.Sdk defaults to MTP, so add <UseVSTest>true</UseVSTest> when preserving VSTest.

Do not change TargetFramework. Remove xunit.runner.json only after porting its relevant settings.

3. Perform the mechanical conversion

Apply the common rewrites first:

Remove using Xunit; and using Xunit.Abstractions;. Add using Microsoft.VisualStudio.TestTools.UnitTesting; for the metapackage option; MSTest.Sdk supplies it as an implicit global using.

Preserve existing class inheritance. Do not mechanically seal classes.

4. Resolve semantic mappings

Load the mapping cheatsheet for every high-risk construct found in Step 1. These rules are mandatory:

• xUnit Assert.Throws<T> is exact-type and maps to MSTest Assert.ThrowsExactly<T>.
• xUnit Assert.ThrowsAny<T> permits derived types and maps to MSTest Assert.Throws<T>.
• xUnit Assert.IsType<T> is exact-type and maps to Assert.IsExactInstanceOfType<T>; Assert.IsAssignableFrom<T> maps to Assert.IsInstanceOfType<T>.
• xUnit Assert.Equal on sequences compares elements. Use Assert.AreSequenceEqual on MSTest 4.3+ or CollectionAssert.AreEqual with materialized lists on earlier v4; never replace sequence equality with reference-based Assert.AreEqual.
• [Ignore] and [Timeout] are modifiers; keep [TestMethod] so the test is discovered.
• [DataRow] values must exactly match parameter types.
• TestContext.Current.CancellationToken maps to an injected MSTest TestContext.CancellationToken; never replace it with CancellationToken.None or a new CancellationTokenSource.
• Owner is a reserved VSTest property. Map [Trait("Owner", value)] to [Owner(value)], not [TestProperty("Owner", value)].
• Assertions with no MSTest equivalent (Assert.Collection, Assert.All, Assert.Equivalent, Record.Exception, event assertions) require an explicit manual rewrite. Never delete an assertion without replacing its verification.

Apply the mechanical and semantic rewrites in one edit pass when the inventory makes the required mappings clear. Do not run an intermediate build by default; use compiler errors from final verification to drive only unresolved conversions.

5. Preserve lifecycle, fixture scope, and parallelization

• Keep constructor setup and IDisposable/IAsyncDisposable when valid. Map IAsyncLifetime to [TestInitialize]/[TestCleanup].
• Map IClassFixture<T> to class-scoped initialization and cleanup.
• For ICollectionFixture<T>, preserve both sharing and serialization. Prefer a static Lazy<T> helper used by each member class; add [DoNotParallelize] only when the source collection disabled parallelization. Use assembly initialization only when the fixture is genuinely assembly-wide.
• Replace ITestOutputHelper with injected or property-based MSTest TestContext.

xUnit runs classes in parallel by default; MSTest runs them serially. Unless the source disabled parallelism, preserve xUnit behavior with:

[assembly: Parallelize(Workers = 0, Scope = ExecutionScope.ClassLevel)]

Never use ExecutionScope.MethodLevel to emulate xUnit. Before applying a fixture-scope or parallelization decision, state what the source shared or serialized and how the target preserves it.

6. Verify parity

1. Run tests once with the same platform, filter, and configuration used for the baseline. dotnet test builds by default; run a separate build only when needed to isolate a compilation failure.
2. Compare discovered, passed, failed, and skipped counts.
3. Investigate every difference before declaring completion:
   • missing cases -> discovery attributes, DynamicData, or DataRow literal types
   • changed exception behavior -> exact-vs-derived assertion mapping
   • shared-state failures or large duration changes -> fixture scope and parallelization
   • silently skipped tests -> missing [TestMethod] or incorrect runtime-skip conversion
4. Confirm no xUnit package, namespace, attribute, runner configuration, or fixture interface remains unless explicitly documented for manual follow-up.

Completion Criteria

• Current xUnit version and test platform were identified
• xUnit packages and source constructs were converted
• Target framework and test platform stayed unchanged
• Fixture scope and parallelization decisions are explicit
• Build succeeds
• Test discovery and result counts match the baseline
• Any unsupported custom extension point is called out rather than approximated

Follow-up

Run migrate-vstest-to-mtp separately if the user also wants MTP. Use writing-mstest-tests only after parity is established to polish the converted MSTest code.