Convert .NET tests from xUnit.net v2/v3 to MSTest v4 while preserving VSTest or MTP. Use for replacing xunit packages, Fact/Theory/InlineData/MemberData, assertions, IClassFixture/ICollectionFixture, ITestOutputHelper, TestContext cancellation, traits/Owner, skips, timeouts, and xUnit parallelization. Also use when a "convert xUnit to MSTest" request may already be migrated: inspect and report the no-op. Do not use for xUnit v2-to-v3, MSTest upgrades, NUnit/TUnit conversion, or runner-only VSTest-to-MTP migration.
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Convert .NET tests from xUnit.net v2/v3 to MSTest v4 while preserving VSTest or MTP. Use for replacing xunit packages, Fact/Theory/InlineData/MemberData, assertions, IClassFixture/ICollectionFixture, ITestOutputHelper, TestContext cancellation, traits/Owner, skips, timeouts, and xUnit parallelization. Also use when a "convert xUnit to MSTest" request may already be migrated: inspect and report the no-op. Do not use for xUnit v2-to-v3, MSTest upgrades, NUnit/TUnit conversion, or runner-only VSTest-to-MTP 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.
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.
Version history
Do not combine this framework conversion with a target-framework upgrade or VSTest/MTP migration. Complete and verify one migration before starting another.
Apply these before the mechanical mapping:
| Detected state | Required action |
|---|---|
| No xUnit package, namespace, attribute, or fixture remains | Stop. Make no file changes, report that migration is unnecessary, and run the existing dotnet test command once to prove the already-MSTest project is healthy. |
| Source uses VSTest | Keep the existing VSTest property/configuration. Prefer retaining and updating a source project's explicit Microsoft.NET.Test.Sdk pin; a repository that intentionally relies on the MSTest metapackage's transitive dependency may keep that convention. Do not introduce MTP properties. |
| Source uses MTP | Replace xUnit-specific MTP selection with MSTest MTP configuration. Prefer MSTest.Sdk; with the metapackage, set EnableMSTestRunner=true and OutputType=Exe. Preserve native-versus-bridged command integration, and do not add <UseVSTest>true</UseVSTest> or other VSTest-only configuration. |
| Source relies on xUnit's default parallelization | Add [assembly: Parallelize(Workers = 0, Scope = ExecutionScope.ClassLevel)] to a compiled .cs file when the current project has at least two independently runnable test classes. In a one-class project with no explicit parallel setting, omit it because class-level concurrency is not observable. Translate explicit CollectionBehavior or xunit.runner.json settings regardless of current class count. Before reporting completion, read the changed file back and name it in the result. |
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.
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:
references/ directory; those files are read-only guidance, not an editing workspacedotnet test --no-restore unless restore is already known to be currentdotnet test is sufficientUse 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.
Directory.Build.props, Directory.Packages.props, global.json, and runner configuration, and search the source for the high-risk constructs below.xunit 2.x and related packages -> xUnit v2xunit.v3 or xunit.v3.* -> xUnit v3platform-detection only when the platform is ambiguous, and preserve the detected platform.IClassFixture, ICollectionFixture, CollectionDefinition, custom FactAttribute/TheoryAttribute/DataAttributeAssert.Throws, ThrowsAny, IsType, Record.Exception, event assertionsITestOutputHelper, TestContext.Current, IAsyncLifetimeCollectionBehavior, xunit.runner.json, shared static or external stateRemove 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:
<!-- Example pin: replace with the exact stable v4 version resolved from the configured package source. -->
<PackageReference Include="MSTest" Version="4.1.0" />
The metapackage includes Microsoft.NET.Test.Sdk, MSTest.TestAdapter, MSTest.TestFramework, and MSTest.Analyzers. When preserving VSTest, prefer retaining a source project's explicit Microsoft.NET.Test.Sdk pin and update it to a version compatible with the chosen MSTest version; this keeps runner/version compatibility reviewable. A repository that intentionally relies on the metapackage's transitive dependency may preserve that convention instead. For the example pin above, MSTest 4.1.0 requires Microsoft.NET.Test.Sdk 18.0.1+; incompatible older pins can cause NU1605.
When preserving MTP, do not carry xUnit's UseMicrosoftTestingPlatformRunner property into the MSTest project. Prefer MSTest.Sdk at the resolved version. If repository conventions require the metapackage route, set <EnableMSTestRunner>true</EnableMSTestRunner> and <OutputType>Exe</OutputType>. Retain TestingPlatformDotnetTestSupport=true only for repositories that continue to invoke MTP applications through VSTest command mode; native .NET 10+ MTP mode does not require it. When preserving VSTest with MSTest.Sdk, set <UseVSTest>true</UseVSTest>.
Do not change TargetFramework. Remove xunit.runner.json only after porting its relevant settings.
Apply the common rewrites first:
| xUnit | MSTest |
|---|---|
| no class attribute | [TestClass] |
[Fact] | [TestMethod] |
[Theory] + [InlineData] | [TestMethod] + [DataRow] |
[MemberData] | [DynamicData] |
[Fact(Skip = "...")] | [TestMethod] + [Ignore("...")] |
[Trait("Category", value)] | [TestCategory(value)] |
[Trait("Owner", value)] | [Owner(value)] |
other [Trait(key, value)] | [TestProperty(key, value)] |
Assert.Equal / NotEqual | Assert.AreEqual / AreNotEqual |
Assert.True / False | Assert.IsTrue / IsFalse |
Assert.Null / NotNull | Assert.IsNull / IsNotNull |
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.
Load the mapping cheatsheet for every high-risk construct found in Step 1. These rules are mandatory:
Assert.Throws<T> is exact-type and maps to MSTest Assert.ThrowsExactly<T>.Assert.ThrowsAny<T> permits derived types and maps to MSTest Assert.Throws<T>.Assert.IsType<T> is exact-type and maps to the generic Assert.IsExactInstanceOfType<T>; Assert.IsAssignableFrom<T> maps to the generic Assert.IsInstanceOfType<T>. When the xUnit assertion's typed return value is assigned, preserve that assignment and use the generic MSTest overload rather than a non-generic Type overload.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)].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.
IDisposable/IAsyncDisposable when valid. Map IAsyncLifetime to [TestInitialize]/[TestCleanup].IClassFixture<T> means one fixture instance per test class, shared by all methods in that class. Map it to a static T field created once by a static [ClassInitialize] method that accepts TestContext, and dispose it once from static [ClassCleanup]. Never use [TestInitialize]/[TestCleanup] for this mapping because that creates one fixture per test method.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.ITestOutputHelper with constructor-injected or property-based MSTest TestContext, and replace each _output.WriteLine(...) call with the corresponding TestContext.WriteLine(...). In the final result, name both the TestContext injection/property and the WriteLine mapping explicitly; "migrated output" is not enough to demonstrate parity.xUnit runs classes in parallel by default; MSTest runs them serially. When the current project has two or more independently runnable test classes, preserve that effective behavior with:
[assembly: Parallelize(Workers = 0, Scope = ExecutionScope.ClassLevel)]
For a one-class project with no explicit xUnit parallel setting, do not add an assembly policy: there is no class-level concurrency to preserve. Always translate explicit CollectionBehavior or xunit.runner.json settings. 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.
dotnet test builds by default; run a separate build only when needed to isolate a compilation failure.DynamicData, or DataRow literal types[TestMethod] or incorrect runtime-skip conversion[ClassInitialize], TestContext.WriteLine, and Assert.IsExactInstanceOfType<T>; generic claims such as "converted attributes" or "migrated output" are not evidence of parity.Keep the final response concise and outcome-focused:
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.
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