migrate-xunit-to-mstest

name	migrate-xunit-to-mstest
description	Migrate .NET test projects from xUnit.net (v2 or v3) to MSTest v4. USE FOR: convert/migrate xUnit tests to MSTest, replace xunit/xunit.v3 packages, port [Fact]/[Theory]/[InlineData]/[MemberData]/[ClassData] to [TestMethod]/[DataRow]/[DynamicData], port Assert.Equal/True/Throws/ThrowsAsync to Assert.AreEqual/IsTrue/ThrowsExactly/ThrowsExactlyAsync, port IClassFixture/ ICollectionFixture/IDisposable/IAsyncLifetime/ITestOutputHelper/[Trait]/[Fact(Skip)] to MSTest equivalents, preserve xUnit parallel-class default via [assembly: Parallelize(Scope = ClassLevel)], remove xunit.runner.json. DO NOT USE FOR: xUnit v2 -> v3 upgrade (use migrate-xunit-to-xunit-v3); MSTest -> xUnit, NUnit/TUnit -> MSTest (no skills exist); MSTest version upgrades (use migrate-mstest-v1v2-to-v3 or migrate-mstest-v3-to-v4); VSTest <-> MTP only (use migrate-vstest-to-mtp); general .NET upgrades.
license	MIT

xUnit -> MSTest Migration

Migrate a .NET test project from xUnit.net (v2 or v3) to MSTest v4. The outcome is a project that:

• References MSTest v4 packages (or MSTest.Sdk 4.x) instead of xunit* / xunit.v3.*
• Has every [Fact]/[Theory] rewritten as [TestMethod] and every assertion mapped to the MSTest equivalent
• Builds cleanly with the same target framework
• Passes the same set of tests (modulo intentional changes documented below)
• Preserves the current test platform (VSTest stays on VSTest; MTP stays on MTP)

This is a cross-framework migration. Do not bundle it with a version upgrade or a platform switch in the same pass -- if both are needed, do this skill first, commit, then run migrate-mstest-v3-to-v4 (if you stopped on v3) or migrate-vstest-to-mtp.

When to Use

• The project references xunit, xunit.assert, xunit.core, xunit.extensibility.core/execution, xunit.abstractions, or any xunit.v3.* package, and you want to switch to MSTest
• You want a single .NET test framework across a solution that today mixes xUnit and MSTest

Inputs

Input	Required	Description
Project or solution path	Yes	The .csproj, .sln, or .slnx containing xUnit test projects
Build command	No	How to build (e.g., dotnet build). Auto-detect if not provided
Test command	No	How to run tests (e.g., dotnet test). Auto-detect if not provided

Response Guidelines

• Always identify the current xUnit version first. State whether the project is on xUnit v2 (xunit 2.x) or xUnit v3 (xunit.v3 / xunit.v3.*) before recommending changes. This grounds the migration advice -- some breaking-change steps only apply to one version.
• Always preserve the current test platform. If the project runs on VSTest, keep VSTest. If it runs on MTP (e.g., xUnit v3 native MTP, or <UseMicrosoftTestingPlatformRunner>true</UseMicrosoftTestingPlatformRunner>), keep MTP. Recommend migrate-vstest-to-mtp as a separate follow-up only if the user asks for it.
• Explicitly communicate every judgement-call decision before applying it -- otherwise the user cannot tell what changed semantically. In particular:
  • Fixture scope changes (Step 8): state the source scope (class / collection / assembly) and the target scope you chose, plus what gets shared and what gets serialized. A silent widening from collection to assembly is the most common way this migration regresses tests.
  • Parallelization (Step 11): state that MSTest defaults to serial execution (xUnit parallelizes classes by default), so an explicit [assembly: Parallelize(...)] is required to match xUnit's behaviour -- omitting it silently halves CI throughput.
  • Assert.Throws<T> -> Assert.ThrowsExactly<T> (Step 6): mention the exact-type-vs-any-derived semantic flip so reviewers know the assertion was deliberately renamed, not just translated.
• Specific API mapping questions (assertions, fixtures, output helper, etc.): jump to the relevant step. Do not run the full workflow.
• Full migration requests: follow the workflow end-to-end.
• Focused fix requests (specific compile error after a partial migration): address only that error using the mapping reference. Do not walk the full workflow.
• Code samples: show concrete before/after using the user's actual type/method names, not generic placeholders.

Strategy

The conversion is mechanical for ~80% of code (attributes and simple assertions) and judgement-based for ~20% (collection fixtures, custom data attributes, exact-type-vs-derived exception assertions, parallelization semantics). Always do the mechanical pass first so build errors point you at the judgement areas.

Mapping Reference

For the full attribute/assertion/fixture/lifecycle mapping tables -- including semantic traps (Assert.Throws<T> vs Assert.ThrowsAny<T>, IClassFixture vs ICollectionFixture scope), edge cases (TheoryData<T...>, MemberType=, custom DataAttribute, custom FactAttribute, Record.Exception), and copy-pasteable before/after snippets -- see references/mapping-cheatsheet.md. Load it whenever you need a specific xUnit -> MSTest equivalent.

For writing idiomatic MSTest code (modern assertion APIs, lifecycle patterns, data-driven conventions, Assert.HasCount/IsEmpty/StartsWith, etc.), see the writing-mstest-tests skill. Do not re-derive idiomatic MSTest patterns here. Apply this skill to convert; apply writing-mstest-tests to polish.

Workflow

Commit strategy: Commit after Step 2 (packages updated, builds broken), after Step 6 (attributes converted, asserts fixed), and after Step 8 (fixtures/lifecycle rewritten, tests pass). Commit before fixing follow-up cleanup so reviewers can bisect.

Step 1: Assess the project

1. Locate every test project. Read .csproj, Directory.Build.props, Directory.Packages.props, and global.json.
2. Identify the xUnit version:
   • xunit 2.x (+ xunit.assert / xunit.core / xunit.abstractions) -> xUnit v2
   • xunit.v3 / xunit.v3.* -> xUnit v3
3. Identify the current test platform (this dictates what to keep, not what to change) by invoking the platform-detection skill. The xUnit/MTP matrix is nuanced -- xunit.v3 inside Test Explorer is MTP by default unless opted out, while xunit.v3 inside dotnet test depends on the xunit.v3.mtp-v* packages -- so do not try to inline a shortcut here. Quick signals to feed into that skill: xunit.runner.visualstudio (v2) usually means VSTest; xunit.v3.mtp-v* / xunit.v3.core.mtp-v* packages or YTest.MTP.XUnit2 (v2 MTP shim) usually mean MTP. <UseMicrosoftTestingPlatformRunner> only affects dotnet run and is not a reliable VSTest-vs-MTP signal on its own.
4. Verify the TargetFramework is supported by MSTest v4:
   • Supported: net8.0, net9.0, net462+, netstandard2.0 (test library only), uap10.0.16299, net8.0-windows10.0.18362.0 (WinUI), net9.0-windows10.0.17763.0 (modern UWP).
   • Unsupported: .NET Core 3.1, net5.0-net7.0. STOP and ask the user to upgrade the TFM first, or migrate to MSTest v3 (then use migrate-mstest-v3-to-v4 after a TFM bump).
5. Inventory high-risk patterns -- scan for these and flag them now so you can plan judgement steps later:
   • Parallelization differences (Step 11) -- xUnit parallelizes test classes by default; MSTest does not. This is the single most common source of post-migration regressions: tests that depended on isolation by parallel scheduling, on the lack of it, or on shared static state can pass differently. Decide the target parallelization model now -- do not leave it as the MSTest default by accident.

Step 2: Replace packages

Choose the package option that matches what the project uses today. When the user says "preserve VSTest" -- or the existing project uses explicit PackageReferences -- default to Option A (MSTest metapackage). Reach for Option B (MSTest.Sdk) only when the user explicitly asks to modernize the SDK or already uses MSTest.Sdk elsewhere in the solution; if you adopt it, you must preserve the platform from Step 1.

Remove every xUnit package reference (from .csproj, Directory.Build.props, Directory.Packages.props):

• xunit, xunit.abstractions, xunit.assert, xunit.core
• xunit.extensibility.core, xunit.extensibility.execution
• xunit.runner.visualstudio
• xunit.v3, xunit.v3.assert, xunit.v3.core, xunit.v3.extensibility.core
• xunit.v3.mtp-v1, xunit.v3.mtp-v2, xunit.v3.core.mtp-v1, xunit.v3.core.mtp-v2
• YTest.MTP.XUnit2 (xUnit v2 MTP shim)
• Companion packages: Xunit.SkippableFact, Xunit.Combinatorial, Xunit.StaFact (see Step 10)

Add MSTest v4. Two options -- both correct.

Option A -- MSTest metapackage (recommended for incremental migrations):

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

The MSTest metapackage pulls in MSTest.TestFramework, MSTest.TestAdapter, MSTest.Analyzers, and Microsoft.NET.Test.Sdk -- so VSTest discovery (vstest.console, classic dotnet test) still works.

MTP code-coverage caveat for Option A: Microsoft.NET.Test.Sdk pulls VSTest's Microsoft.CodeCoverage transitively. If the project from Step 1 is on MTP and uses code coverage, that transitive dependency can interfere with MTP's collector (Microsoft.Testing.Extensions.CodeCoverage). Prefer Option B (MSTest.Sdk without UseVSTest) for MTP projects -- the SDK omits Microsoft.NET.Test.Sdk and wires the MTP coverage collector instead. If you must stay on Option A for an MTP project, verify coverage works on a representative test run before merging.

Option B -- MSTest.Sdk:

<Project Sdk="MSTest.Sdk/4.1.0">
  <PropertyGroup>
    <!-- Keep the project's existing TargetFramework -- do NOT retarget during migration. -->
    <TargetFramework>$(ExistingTargetFramework)</TargetFramework>
  </PropertyGroup>
</Project>

MSTest.Sdk defaults to MTP. To preserve a VSTest project, opt back in with <UseVSTest>true</UseVSTest> -- the SDK then pulls in Microsoft.NET.Test.Sdk automatically (no extra PackageReference needed):

<PropertyGroup>
  <UseVSTest>true</UseVSTest>
</PropertyGroup>

For solutions with several test projects, prefer pinning the MSTest.Sdk version in global.json so it lives in one place:

{
  "msbuild-sdks": {
    "MSTest.Sdk": "4.1.0"
  }
}

With the pin in global.json, the project line simplifies to <Project Sdk="MSTest.Sdk">.

When switching to MSTest.Sdk, also remove now-redundant properties: <OutputType>Exe</OutputType>, <IsPackable>false</IsPackable>, <IsTestProject>true</IsTestProject>, <EnableMSTestRunner>.

MSTest.Sdk also adds Microsoft.VisualStudio.TestTools.UnitTesting as an implicit global using. Do not add <Using Include="Microsoft.VisualStudio.TestTools.UnitTesting" /> to the project (it's noise) and skip the per-file using Microsoft.VisualStudio.TestTools.UnitTesting; in Step 4 -- you only need it for projects on Option A (the MSTest metapackage).

Step 3: Update project configuration

1. Preserve the runner. Confirm the platform decision from Step 1 still holds after Step 2. Common mistakes:
   • Switching to MSTest.Sdk without UseVSTest=true silently flips a VSTest project to MTP. Add <UseVSTest>true</UseVSTest> to the project (the SDK pulls in Microsoft.NET.Test.Sdk automatically -- no manual PackageReference needed).
   • <UseMicrosoftTestingPlatformRunner>true</UseMicrosoftTestingPlatformRunner> only affects the dotnet run entry point and is not a runner switch in Test Explorer or dotnet test. Do not infer the platform from this property in either direction -- defer to the platform-detection skill (see Step 1).
2. Delete xunit.runner.json and port any settings you need (parallelization, [CollectionBehavior], appDomain) per Step 11's "xunit.runner.json -> MSTest" sub-table. The settings have no direct MSBuild-property mapping.
3. Remove using Xunit; and using Xunit.Abstractions; from C# files. For Option A (MSTest metapackage), add using Microsoft.VisualStudio.TestTools.UnitTesting; per file (Step 4 covers this alongside the other rewrites). For Option B (MSTest.Sdk), skip the per-file using -- the SDK provides it as an implicit global using.

Step 4: Convert test classes and methods

Apply these rewrites to every C# test file. Class-level first, then method-level.

Class:

• Add [TestClass] to every class that contained xUnit [Fact]/[Theory] methods (xUnit had no class-level requirement).
• Preserve the original class hierarchy. xUnit projects often use base/derived test classes (shared setup, helper assertions, generic base fixtures); marking classes sealed would break that pattern. Sealing is an optional follow-up handled by writing-mstest-tests, not part of the mechanical migration.
• Replace using Xunit; / using Xunit.Abstractions; with using Microsoft.VisualStudio.TestTools.UnitTesting;. On Option B (MSTest.Sdk), skip adding the MSTest using -- the SDK provides it as an implicit global using, so just remove the using Xunit; / using Xunit.Abstractions; lines (Step 2 and Step 3 cover this).

Methods:

[Ignore] and [Timeout] are modifiers, not discovery attributes. Always emit [TestMethod] alongside them -- a method with [Ignore] but no [TestMethod] is silently skipped by the test runner (no error, no skip count). Same for [Timeout].

xUnit	MSTest
[Fact]	[TestMethod]
[Theory]	[TestMethod] (parameterized; MSTest 3+ no longer needs [DataTestMethod])
[Fact(DisplayName = "x")]	[TestMethod("x")] (v3 of MSTest) or [TestMethod(DisplayName = "x")] (v4)
[Fact(Skip = "reason")]	[TestMethod] + [Ignore("reason")] (both attributes required)
[Fact(Timeout = 5000)]	[TestMethod] + [Timeout(5000)] (both attributes required)
[Trait("Category", "Unit")]	[TestCategory("Unit")]
[Trait("Owner", "alice")]	[TestProperty("Owner", "alice")]

Both [TestCategory] and [TestProperty] are filterable at runtime (--filter "TestCategory=Unit" / --filter "Owner=alice"). [TestCategory] targets Assembly, Class, and Method, so an xUnit [assembly: Trait("Category", ...)] keeps its assembly scope under MSTest as [assembly: TestCategory(...)]. [TestProperty] targets only Class and Method — there is no AttributeTargets.Assembly, so an assembly-level xUnit trait with an arbitrary key must collapse to [assembly: TestCategory(...)] (or be pushed down to every class). Use [TestCategory] for the conventional category trait; use [TestProperty] for arbitrary key/value metadata at class/method scope. For environmental skips (OS-specific, CI-only), MSTest 3.10+'s [OSCondition] / [CICondition] are usually a better fit than overloading a trait -- see Step 6 / cheatsheet §3.9.

Step 5: Convert data-driven tests

xUnit	MSTest
[InlineData(1, 2)]	[DataRow(1, 2)]
[InlineData(1, DisplayName = "case 1")]	[DataRow(1, DisplayName = "case 1")]
[MemberData(nameof(Cases))] returning IEnumerable<object[]>	[DynamicData(nameof(Cases))] returning IEnumerable<object[]>
[MemberData(nameof(Cases), MemberType = typeof(X))]	[DynamicData(nameof(Cases), typeof(X))]
[MemberData(nameof(Method), arg1, arg2)] (parameterized member)	Manual: convert to a parameterless property or compute the inputs inside the test
[ClassData(typeof(MyData))] (class implementing IEnumerable<object[]>)	Add a static property => new MyData() on the test class, then [DynamicData(nameof(Cases))]
TheoryData<int, string>	IEnumerable<object[]>, IEnumerable<(int, string)> (MSTest 3.7+ ValueTuple), or IEnumerable<TestDataRow<(int, string)>> (strongly-typed with per-row metadata)
Custom DataAttribute subclass	Manual: implement ITestDataSource (GetData, GetDisplayName)

Prefer ValueTuple data sources for new MSTest tests (see writing-mstest-tests), but for migration keep IEnumerable<object[]> -- it minimizes diff churn and works in both MSTest 3 and 4.

Step 6: Convert assertions

Most common cases inline. For the full table including string/collection/type/numeric and event/equivalence assertions, see references/mapping-cheatsheet.md §3.

xUnit	MSTest
Assert.Equal(expected, actual)	Assert.AreEqual(expected, actual)
Assert.NotEqual(a, b)	Assert.AreNotEqual(a, b)
Assert.True(x) / Assert.False(x)	Assert.IsTrue(x) / Assert.IsFalse(x)
Assert.Null(x) / Assert.NotNull(x)	Assert.IsNull(x) / Assert.IsNotNull(x)
Assert.Same(a, b) / Assert.NotSame(a, b)	Assert.AreSame(a, b) / Assert.AreNotSame(a, b)
Assert.Throws<T>(() => ...)	Assert.ThrowsExactly<T>(() => ...) (see trap below)
Assert.ThrowsAny<T>(() => ...)	Assert.Throws<T>(() => ...)
await Assert.ThrowsAsync<T>(...)	await Assert.ThrowsExactlyAsync<T>(...)
Assert.IsType<T>(x) (exact-type check, returns T)	Assert.IsExactInstanceOfType<T>(x) (MSTest 4.1+, returns T) -- not Assert.IsInstanceOfType<T>, which is assignable/is-a and silently weakens the assertion
Assert.IsNotType<T>(x) (exact-type check)	Assert.IsNotExactInstanceOfType<T>(x) (MSTest 4.1+)
Assert.IsAssignableFrom<T>(x)	Assert.IsInstanceOfType<T>(x) (MSTest v4 returns the typed value)
Assert.Empty(coll) / Assert.NotEmpty(coll)	Assert.IsEmpty(coll) / Assert.IsNotEmpty(coll)
Assert.Single(coll)	var item = Assert.ContainsSingle(coll);

Critical semantic trap -- exception assertions:

• xUnit Assert.Throws<T> = exact type match -> MSTest Assert.ThrowsExactly<T>.
• xUnit Assert.ThrowsAny<T> = derived types also match -> MSTest Assert.Throws<T>.

Reversing these flips the assertion semantics silently. Verify by name, not by visual similarity.

No-equivalent assertions -- convert manually (see cheatsheet §3.11):

• Assert.Collection(items, e1 => ..., e2 => ...) -> assert count, then per-element
• Assert.All(items, x => ...) -> foreach
• Assert.Equivalent(expected, actual) -> deep equality manually, or a third-party library
• Assert.Raises<T> / Assert.PropertyChanged -> manual event subscription + flag check
• Record.Exception / Record.ExceptionAsync -> try/catch returning the exception (or Assert.ThrowsExactly<T> if you know the type)

Step 7: Convert lifecycle

Constructor / IDisposable / IAsyncDisposable / IAsyncLifetime:

xUnit	MSTest
Constructor (sync setup)	Keep constructor (MSTest also instantiates per test). Drop xUnit-only ITestOutputHelper param -- see Step 9
Dispose() (sync teardown)	Keep Dispose() (MSTest supports IDisposable) or rewrite as [TestCleanup] public void Cleanup() { ... }
DisposeAsync() (async teardown)	Keep IAsyncDisposable.DisposeAsync() or rewrite as [TestCleanup] public async Task CleanupAsync() { ... }
IAsyncLifetime.InitializeAsync	[TestInitialize] public async Task InitAsync() { ... }
IAsyncLifetime.DisposeAsync	[TestCleanup] public async Task CleanupAsync() { ... }

Per writing-mstest-tests: prefer the constructor for sync init (it allows readonly fields). Use [TestInitialize] only for async setup or when you need TestContext.

Step 8: Convert fixtures (high-risk -- read carefully)

IClassFixture<T> -- class-level shared state (mechanical):

// xUnit v2/v3
public class DbFixture : IDisposable
{
    public string ConnectionString { get; } = "...";
    public void Dispose() { /* cleanup */ }
}

public class OrderTests : IClassFixture<DbFixture>
{
    private readonly DbFixture _fixture;
    public OrderTests(DbFixture fixture) => _fixture = fixture;
}

// MSTest equivalent
[TestClass]
public sealed class OrderTests
{
    private static DbFixture? s_fixture;

    [ClassInitialize]
    public static void ClassInit(TestContext context) => s_fixture = new DbFixture();

    [ClassCleanup]
    public static void ClassCleanup() => s_fixture?.Dispose();
}

ICollectionFixture<T> / [CollectionDefinition] -- shared by tests in the same collection (judgement call):

xUnit collections do two things simultaneously: (1) share a fixture instance across multiple test classes, and (2) serialize those classes (no parallel execution within a collection). MSTest does not have a built-in equivalent that preserves both semantics. Pick one -- do not silently map to [AssemblyInitialize]:

• Few classes, narrow scope: copy the fixture initialization into each class's [ClassInitialize], OR introduce a static Lazy<T> shared helper. Add [DoNotParallelize] on each class to preserve serialization.
• Many classes, fixture is genuinely assembly-wide (e.g., process-wide TestServer): hoist to [AssemblyInitialize] / [AssemblyCleanup] in a dedicated AssemblySetup class and confirm with the user that widening the scope is acceptable. Note that this changes parallelization semantics.
• Custom collection behavior or test-collection-orderer: stop and flag for manual review.

REQUIRED -- communicate the scope decision before applying it. Silently widening fixture scope across the assembly is the most common way this migration regresses tests. Use this template (replace bracketed text):

"The xUnit [Collection(\"<name>\")] shared a <Fixture> between <N> classes and serialized them. I am mapping that to: a static Lazy<<Fixture>> shared by each class's [ClassInitialize] (scope: per-class, shared via static -- not widened to assembly), plus [DoNotParallelize] on <ClassA> and <ClassB> to preserve the serialization. The alternative -- [AssemblyInitialize] -- would widen the fixture to every test in the assembly, which I rejected because <reason>."

Step 9: Convert output and TestContext

ITestOutputHelper -> TestContext:

// xUnit (v2 and v3)
public class MyTests
{
    private readonly ITestOutputHelper _output;
    public MyTests(ITestOutputHelper output) => _output = output;

    [Fact]
    public void Test() => _output.WriteLine("...");
}

// MSTest (v3.6+ supports TestContext in constructor)
[TestClass]
public sealed class MyTests
{
    private readonly TestContext _testContext;
    public MyTests(TestContext testContext) => _testContext = testContext;

    [TestMethod]
    public void Test() => _testContext.WriteLine("...");
}

If the project pins MSTest < 3.6 (rare after Step 2), use property injection instead:

public TestContext TestContext { get; set; } = null!;

xUnit v3 TestContext.Current (TestContext.Current is static in xUnit v3; in MSTest you must use the instance TestContext obtained via the same constructor or property injection shown above):

• TestContext.Current.CancellationToken -> _testContext.CancellationToken (MSTest 3.6+)
• TestContext.Current.AddAttachment(name, path) -> _testContext.AddResultFile(path)
• TestContext.Current.TestOutputHelper.WriteLine(...) -> _testContext.WriteLine(...)

REQUIRED for CancellationToken: Add the constructor injection from above (or property injection if pinned to MSTest < 3.6) even if the class only uses TestContext.Current.CancellationToken (no ITestOutputHelper). Do NOT replace TestContext.Current.CancellationToken with CancellationToken.None or a freshly-constructed CancellationTokenSource -- both lose the test-host's cancellation linkage and change behavior under timeouts.

// xUnit v3
[Fact]
public async Task WorkRespectsCancellation()
{
    var ct = TestContext.Current.CancellationToken;
    await Task.Delay(1, ct);
    Assert.False(ct.IsCancellationRequested);
}

// MSTest (note: Assert.False -> Assert.IsFalse from Step 6)
[TestClass]
public sealed class MyTests
{
    private readonly TestContext _testContext;
    public MyTests(TestContext testContext) => _testContext = testContext;

    [TestMethod]
    public async Task WorkRespectsCancellation()
    {
        var ct = _testContext.CancellationToken;
        await Task.Delay(1, ct);
        Assert.IsFalse(ct.IsCancellationRequested);
    }
}

Step 10: Convert companion packages

xUnit companion	MSTest equivalent
Xunit.SkippableFact ([SkippableFact], Skip.If, Skip.IfNot)	For environmental predicates (OS/CI/arch): MSTest 3.10+ condition attributes ([OSCondition], [CICondition], etc.). Otherwise: [Ignore] (compile-time) or Assert.Inconclusive("reason") (runtime). Remove the package
Xunit.Combinatorial ([CombinatorialData], [CombinatorialValues])	Combinatorial.MSTest (community port; attribute surface matches xUnit.Combinatorial). Or expand combinations into explicit [DataRow]s / [DynamicData]
Xunit.StaFact ([StaFact], [WpfFact])	[TestMethod] + manual STA thread. No MSTest equivalent for [WpfFact]; flag for manual conversion
Verify.Xunit	Verify.MSTest -- swap the package; usage is similar
FluentAssertions / Shouldly / AwesomeAssertions	Keep -- assertion library is framework-agnostic
Moq / NSubstitute / FakeItEasy	Keep -- mocking library is framework-agnostic

Step 11: Handle parallelization (defaults differ -- read carefully)

This is the most common source of post-migration regressions. xUnit and MSTest have opposite defaults. Do not skip this step even if Step 1 said tests passed cleanly.

How each framework parallelizes by default

Framework	Across test classes	Within a test class	Test-class instance lifetime
xUnit v2	Parallel (one class per worker thread)	Serial (one test method at a time)	New instance per test method
xUnit v3	Parallel (same as v2)	Serial (same as v2)	New instance per test method
MSTest (default)	Serial (one class at a time)	Serial (one test method at a time)	New instance per test method
MSTest + [assembly: Parallelize(Scope = ClassLevel)]	Parallel	Serial	Same
MSTest + [assembly: Parallelize(Scope = MethodLevel)]	Parallel	Parallel -- more aggressive than xUnit	Same

Workers = 0 means "use all available logical cores" (MSTest's recommended default for parallel runs); any positive integer caps the worker count.

Pick a target model -- there are three reasonable choices

Choice A -- Match xUnit's behaviour exactly (recommended default):

// Place in any .cs file at assembly scope (often AssemblyInfo.cs or GlobalUsings.cs)
[assembly: Parallelize(Workers = 0, Scope = ExecutionScope.ClassLevel)]

Use this when the suite was healthy on xUnit and you want zero behavioural change. It preserves "parallel across classes, serial within a class" exactly.

REQUIRED -- explicitly tell the user why this attribute is needed. When applying Choice A, include this sentence (verbatim or near-verbatim) in your final summary:

"MSTest defaults to serial execution across classes (unlike xUnit, which parallelizes classes by default), so this [assembly: Parallelize(Workers = 0, Scope = ExecutionScope.ClassLevel)] is required to match the project's previous xUnit parallel-class behaviour. Without it, the suite would still pass but run roughly one-class-at-a-time and CI throughput would drop."

The user must understand this is opt-in under MSTest -- a silent omission looks like a no-op but is actually a behavioural regression.

Choice B -- Adopt MSTest's serial default:

// No [assembly: Parallelize] needed -- this is the default

Use this only when the suite has known shared-state issues (Step 1.6) that you intend to leave unfixed for now, or when wall-clock time is not a concern. Expect significantly slower CI.

Choice C -- Selective parallelization:

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

Plus per-class opt-out for the classes that genuinely cannot run concurrently:

[TestClass]
[DoNotParallelize]
public sealed class DatabaseIntegrationTests { /* ... */ }

Use this when most of the suite is isolated but a few classes touch shared state (one DB, fixed ports, file system locations). This is usually the right answer when migrating from xUnit collections.

Do not pick ExecutionScope.MethodLevel to "match xUnit" -- it parallelizes test methods within a single class, which xUnit never does. It is more aggressive than xUnit and will surface latent intra-class state issues.

Translate xUnit parallelization opt-outs

xUnit pattern	MSTest equivalent
[assembly: CollectionBehavior(DisableTestParallelization = true)]	Omit [assembly: Parallelize] (or use Choice B above)
[assembly: CollectionBehavior(MaxParallelThreads = N)]	[assembly: Parallelize(Workers = N, Scope = ExecutionScope.ClassLevel)]
[Collection("Db")] on multiple classes (forces those classes to share a fixture and run serially)	[DoNotParallelize] on each of those classes (preserves serialization) + Step 8 fixture handling (preserves sharing)
[CollectionDefinition("Db", DisableParallelization = true)]	Same as above -- [DoNotParallelize] on each member class
[Collection("Foo")] used only for fixture sharing (no parallelization concern)	Step 8 fixture handling; do not add [DoNotParallelize]

The distinction in the last two rows matters: xUnit collections conflate "share state" with "serialize". MSTest decouples them. Read the original [CollectionDefinition] carefully -- if DisableParallelization is false (or omitted), only the fixture sharing semantic needs to migrate, not the serialization.

Verify after Step 13

If pass/fail counts diverge from the baseline after migration, parallelization is the first place to look:

• More failures than baseline: tests are now running concurrently and stomping shared state. Either add [DoNotParallelize] to the offending classes, or fix the shared state.
• Fewer failures than baseline (tests previously flaky now green): probably means a race condition that xUnit's scheduling exposed is now hidden by serial execution. Note it in a follow-up issue -- do not declare victory.
• Same count but tests take much longer: you forgot [assembly: Parallelize]. Add Choice A.
• Same count but tests take much less time and occasionally fail: you picked MethodLevel instead of ClassLevel. Switch to ClassLevel.

Other runner config: xunit.runner.json migration

Delete xunit.runner.json. Port relevant settings:

xunit.runner.json	MSTest equivalent
"parallelizeAssembly": false	Default in MSTest -- no action
"parallelizeTestCollections": false	Omit [assembly: Parallelize] (Choice B)
"maxParallelThreads": N	[assembly: Parallelize(Workers = N, Scope = ExecutionScope.ClassLevel)]
"methodDisplay": "method" / "classAndMethod"	No equivalent (MSTest always uses class + method)
"diagnosticMessages": true	Use --diagnostic on the CLI, or set verbosity in .runsettings
"preEnumerateTheories": false	No equivalent (MSTest enumerates [DataRow]/[DynamicData] eagerly)
"longRunningTestSeconds": N	Use [Timeout(N * 1000)] per test
"appDomain": "denied" / "ifAvailable"	No equivalent (MSTest uses no app domains on modern .NET)

If the project uses xUnit traits in CI filter expressions (e.g., --filter "Category=Unit" with xUnit), the equivalent MSTest filter is --filter "TestCategory=Unit" (VSTest) or --filter-trait "TestCategory=Unit" (MTP). Update CI pipelines accordingly.

Step 12: Convert xUnit assembly attributes

Some xUnit assembly attributes have direct MSTest equivalents at assembly scope; others must be removed (and re-applied per class/method) or reimplemented against MSTest extensibility.

Convert (assembly scope preserved):

• [assembly: Xunit.Trait("Category", "v")] -> [assembly: TestCategory("v")] -- TestCategoryAttribute targets Assembly, Class, and Method; assembly application propagates to every test.

Convert (assembly scope NOT preserved):

• [assembly: Xunit.Trait("k", "v")] (non-category key) -> collapse to [assembly: TestCategory("v")] if the value alone is sufficient as a filter, or move the trait down to every test class as [TestProperty("k", "v")]. TestPropertyAttribute only targets Class and Method (no AttributeTargets.Assembly) -- [assembly: TestProperty(...)] will not compile.

Delete (no MSTest equivalent or now handled elsewhere):

• [assembly: CollectionBehavior(...)] -- replaced by [assembly: Parallelize(...)] (Step 11)
• [assembly: TestCaseOrderer(...)] -- reimplement against MSTest extensibility; flag for manual conversion
• [assembly: TestCollectionOrderer(...)] -- flag for manual conversion
• [assembly: TestFramework(...)]
• [assembly: CaptureConsole] (xUnit v3) -- MSTest does not capture console by default

Custom orderers/test framework hooks must be reimplemented against MSTest's extensibility model (TestMethodAttribute subclasses, ITestDataSource, etc.) -- stop and flag for manual conversion if present.

Step 13: Build and verify parity

1. dotnet build -- must succeed with zero errors. Address remaining errors using the mapping reference.
2. Invoke the run-tests skill to test the same project, framework, and filter as the Step 1.7 baseline.
3. Compare pass/fail counts to the baseline from Step 1.7. Investigate any deltas:
   • New failures on shared-state tests -- you enabled parallelization (Choice A/C in Step 11) and tests are now stomping each other. Add [DoNotParallelize] to the specific class(es), or fix the shared state.
   • Tests previously parallel now serial (wall-clock much longer) -- you forgot [assembly: Parallelize]. See Step 11 Choice A.
   • Tests previously flaky now consistently green -- almost certainly a race condition hidden by MSTest's serial default. Open a follow-up issue; do not declare victory.
   • Tests now skipped ([Ignore]) that used to run via Assert.SkipWhen? Convert to runtime Assert.Inconclusive if you want them to execute when the condition is false.
   • Theory cases dropped? Check [DataRow] literal types (1 int vs 1L long -- MSTest enforces exact match unlike xUnit).
   • Tests passing but executing 0 assertions? Likely an Assert.Collection or Assert.All was dropped -- restore manually.
4. After parity is confirmed, run the test-quality skills (test-anti-patterns, assertion-quality) to identify follow-up improvements -- e.g., replacing Assert.IsTrue(x.Count() == 3) with Assert.HasCount(3, x).

Validation

• No xunit*, xunit.v3.*, or YTest.MTP.XUnit2 package references remain
• Every test class has [TestClass] and every test method has [TestMethod]
• using Xunit; and using Xunit.Abstractions; removed
• xunit.runner.json removed; equivalent config in .runsettings / [assembly: Parallelize]
• Parallelization is explicit -- either [assembly: Parallelize(...)] is present (Choice A/C, matches xUnit default) or the user accepted the serial default (Choice B). Not left unspecified by accident
• Project builds with zero errors
• Same number of tests discovered as before migration (-- not silently dropping data rows or skipped tests)
• Same pass/fail count as the pre-migration baseline
• Test platform unchanged (VSTest stayed VSTest, MTP stayed MTP) unless the user requested otherwise
• TargetFramework unchanged unless MSTest v4 forced an upgrade (and the user approved)

Common Pitfalls

Pitfall	Symptom	Fix
Leaving parallelization unspecified	Suite that ran in 30s on xUnit now takes minutes on MSTest; or new flakiness from inherited xUnit assumptions	Pick a target parallelization model explicitly in Step 11 (Choice A matches xUnit) -- do not leave it as the MSTest serial default by accident
Picking ExecutionScope.MethodLevel to "match xUnit"	New flakiness on tests sharing instance state within a class	Use ExecutionScope.ClassLevel -- it matches xUnit exactly
Mapping Assert.Throws<T> to Assert.Throws<T>	Tests pass for derived exception types they shouldn't	Map xUnit Assert.Throws<T> to MSTest Assert.ThrowsExactly<T>
Silently widening ICollectionFixture to assembly scope	State leak between unrelated tests; new flakiness	Step 8 -- pick scope explicitly and disclose to the user
MSTest.Sdk flipping VSTest project to MTP	vstest.console finds zero tests; CI breaks	Add <UseVSTest>true</UseVSTest> (no separate Microsoft.NET.Test.Sdk package needed -- the SDK pulls it in)
[DataRow] type mismatch	Theory cases compile in xUnit but produce MSTest runtime errors	Use exact literal types: 1 int, 1L long, 1.0f float
Assert.SkipUnless becomes [Ignore]	Tests that would have run on this machine now silently skip everywhere	Use a condition attribute ([OSCondition]/[CICondition], MSTest 3.10+) when the predicate is environmental; otherwise runtime Assert.Inconclusive
Dropping Assert.Collection / Assert.All without replacement	Test passes but verifies nothing	Restore as explicit foreach + per-element assertions
Leaving xunit.runner.json in the project	Build warning + dead config

Next Steps

After this migration:

• Run migrate-vstest-to-mtp if you want to move to Microsoft.Testing.Platform (separate, committable migration).
• Run writing-mstest-tests to polish converted code: replace Assert.IsTrue(x.Count() == 3) with Assert.HasCount(3, x), prefer ValueTuple data sources, mark classes sealed, etc.
• Run test-anti-patterns / assertion-quality to catch any quality regressions introduced by mechanical conversion.