migrate-mstest-v3-to-v4

Imported from dotnet/skills (plugins/dotnet-test-migration/skills/migrate-mstest-v3-to-v4/SKILL.md) via skills.sh. Install upstream with npx skills add dotnet/skills --skill migrate-mstest-v3-to-v4. Copyright stays with the author (MIT).

MSTest v3 -> v4 Migration

Migrate a test project from MSTest v3 to MSTest v4. The outcome is a project using MSTest v4 that builds cleanly, passes tests, and accounts for every source-incompatible and behavioral change. MSTest v4 is not binary compatible with MSTest v3 -- any library compiled against v3 must be recompiled against v4.

First Action

Inspect the supplied project and source before searching the web or answering from memory. Classify the request as a focused source fix, runtime behavior change, CI discovery issue, compatibility question, or full migration, then follow the matching row below. A clean compile does not exclude this skill: discovery failures, TestContext lifecycle exceptions, and test-history changes are runtime migration failures.

When to Use

• Upgrading MSTest.TestFramework, MSTest.TestAdapter, or MSTest metapackage from 3.x to 4.x
• Upgrading MSTest.Sdk from 3.x to 4.x
• Fixing build errors after updating to MSTest v4 packages
• Resolving behavioral changes in test execution after upgrading to MSTest v4
• Updating custom TestMethodAttribute or ConditionBaseAttribute implementations for v4

When Not to Use

• The project already uses MSTest v4 and builds cleanly -- migration is done
• Upgrading from MSTest v1 or v2 -- use migrate-mstest-v1v2-to-v3 first, then return here
• The project does not use MSTest
• Migrating between test frameworks (e.g., MSTest to xUnit or NUnit)

Inputs

Decisions That Change the Result

Response Guidelines

• Always identify the current version first: Before recommending any migration steps, explicitly state the current MSTest version detected in the project (e.g., "Your project uses MSTest v3 (3.8.0)"). This confirms you've read the project files and grounds the migration advice.
• Resolve, do not assume, the target version: When the user asks for "latest", query the project's configured package source and select the latest stable MSTest v4 version available at execution time. Never copy the example version from this skill into the result without checking it. Keep all MSTest packages on the same resolved version.
• Focused fix requests (user has specific compilation errors after upgrading): Address only the relevant breaking changes from Step 3. Make edits only when the requested deliverable is a source change; "how do I fix these?" remains an answer request. Always provide concrete fixed code using the user's actual types and method names. If the fixture still references v3, do not claim that a green v3 build verifies v4 compatibility; either update packages when requested or state the verification boundary. For custom TestMethodAttribute subclasses, show the full fixed class including CallerInfo propagation to the base constructor. Mention any related analyzer that could have caught this earlier (e.g., MSTEST0006 for ExpectedException). When the project uses MSTest.Sdk, also state that v4 MTP mode no longer supplies Microsoft.NET.Test.Sdk and whether that affects the visible runner. Do not walk through the entire migration workflow.
• "What to expect" questions (user asks about breaking changes before upgrading): Present ALL major breaking changes from the Step 3 quick-lookup table -- not just the ones visible in the current code. For each, provide a one-line fix summary. Also mention key behavioral changes from Step 4 (especially TestCase.Id history impact and TreatDiscoveryWarningsAsErrors default). If project code is available, highlight which changes apply directly.
• Full migration requests (user wants complete migration): Follow the complete workflow below.
• Behavioral/runtime symptom reports (user describes test execution differences without build errors): Match described symptoms to the behavioral changes table in Step 4. Provide targeted, symptom-specific advice. Mention other behavioral changes the user should watch for. Do not walk through source breaking changes unless the user also has build errors.
• CI/test-discovery issues (tests not discovered, vstest.console stopped working, CI pipeline failures after upgrading): Focus on 4.5 (MSTest.Sdk v4 no longer includes Microsoft.NET.Test.Sdk in its default MTP mode -- it is still needed for vstest.console) and 4.4 (TreatDiscoveryWarningsAsErrors). Explain the root cause clearly and give all three paths: add Microsoft.NET.Test.Sdk while preserving MTP, set UseVSTest to switch the project runner, or switch CI to MTP-native dotnet test. Do not walk through the full migration workflow.
• Explanatory questions (user asks "is this a known change?", "what else should I watch out for?"): Explain the relevant changes and advise. Mention related changes the user might encounter next. Do not prescribe a full migration procedure.
• Result proof: End implementation work with the detected v3 version, resolved v4 version, runner choice, files changed, and actual build/test counts. Never report a build, VSTest compatibility, discovery, or passing tests from inference.

Workflow

Commit strategy: Do not create commits unless the user asks. Keep package, source, and behavioral changes logically separable in the diff, but finish and verify the requested migration.

Step 1: Assess the project

1. Identify the current MSTest version by checking package references for MSTest, MSTest.TestFramework, MSTest.TestAdapter, or MSTest.Sdk in .csproj, Directory.Build.props, or Directory.Packages.props.
2. Confirm the project is on MSTest v3 (3.x). If on v1 or v2, use migrate-mstest-v1v2-to-v3 first.
3. Check target framework(s) -- MSTest v4 drops support for .NET Core 3.1 through .NET 7. Supported target frameworks are: net8.0, net9.0, net462 (.NET Framework 4.6.2+), uap10.0.16299 (UWP), net9.0-windows10.0.17763.0 (modern UWP), and net8.0-windows10.0.18362.0 (WinUI).
4. Check for custom TestMethodAttribute subclasses -- these require changes in v4.
5. Check for usages of ExpectedExceptionAttribute -- removed in v4 (deprecated since v3 with analyzer MSTEST0006).
6. Check for usages of Assert.ThrowsException (deprecated) -- removed in v4.
7. Run a clean build to establish a baseline of existing errors/warnings.

Step 2: Update packages to MSTest v4

First resolve the latest stable v4 version from the configured package source. Pin that exact version consistently in the metapackage, individual packages, MSTest.Sdk, and central package management.

• For the MSTest metapackage, update its PackageReference to the resolved exact version.
• For individual packages, update MSTest.TestFramework and MSTest.TestAdapter to that same version.
• For MSTest.Sdk, update the SDK version in the project or global.json pin to that same version.

Run dotnet restore, then dotnet build. Collect all errors for Step 3.

Step 3: Resolve source breaking changes

Work through compilation errors systematically. Use this quick-lookup table to identify all applicable changes, then apply each fix:

Important: Scan the entire project for ALL patterns above before starting fixes. Multiple breaking changes often coexist in the same project.

3.1 TestMethodAttribute.Execute -> ExecuteAsync

If you have custom TestMethodAttribute subclasses that override Execute, change to ExecuteAsync. This change was made because the v3 synchronous Execute API caused deadlocks when test code used async/await internally -- the synchronous wrapper would block the thread while the async operation needed that same thread to complete.

// Before (v3)
public sealed class MyTestMethodAttribute : TestMethodAttribute
{
    public override TestResult[] Execute(ITestMethod testMethod)
    {
        // custom logic
        return result;
    }
}

// After (v4) -- Option A: wrap synchronous logic with Task.FromResult
public sealed class MyTestMethodAttribute : TestMethodAttribute
{
    public override Task<TestResult[]> ExecuteAsync(ITestMethod testMethod)
    {
        // custom logic (synchronous)
        return Task.FromResult(result);
    }
}

// After (v4) -- Option B: make properly async
public sealed class MyTestMethodAttribute : TestMethodAttribute
{
    public override async Task<TestResult[]> ExecuteAsync(ITestMethod testMethod)
    {
        // custom async logic
        return await base.ExecuteAsync(testMethod);
    }
}

Use Task.FromResult when your override logic is purely synchronous. Use async/await when you call base.ExecuteAsync or other async methods.

3.2 TestMethodAttribute CallerInfo constructor

TestMethodAttribute now uses [CallerFilePath] and [CallerLineNumber] parameters in its constructor.

If you inherit from TestMethodAttribute, propagate caller info to the base class:

public class MyTestMethodAttribute : TestMethodAttribute
{
    public MyTestMethodAttribute(
        [CallerFilePath] string callerFilePath = "",
        [CallerLineNumber] int callerLineNumber = -1)
        : base(callerFilePath, callerLineNumber)
    {
    }
}

If the subclass has its own display-name constructor, do not pass that string to the v4 base constructor. Propagate only caller information and assign the DisplayName property:

public sealed class NamedTestMethodAttribute : TestMethodAttribute
{
    public NamedTestMethodAttribute(
        string displayName,
        [CallerFilePath] string callerFilePath = "",
        [CallerLineNumber] int callerLineNumber = -1)
        : base(callerFilePath, callerLineNumber)
    {
        DisplayName = displayName;
    }
}

If you use [TestMethodAttribute("Custom display name")], switch to the named parameter syntax:

// Before (v3)
[TestMethodAttribute("Custom display name")]

// After (v4)
[TestMethodAttribute(DisplayName = "Custom display name")]

3.3 ClassCleanupBehavior enum removed

The ClassCleanupBehavior enum is removed. In v3, this enum controlled whether class cleanup ran at end of class (EndOfClass) or end of assembly (EndOfAssembly). In v4, class cleanup always runs at end of class. Remove the enum argument:

// Before (v3)
[ClassCleanup(ClassCleanupBehavior.EndOfClass)]
public static void ClassCleanup(TestContext testContext) { }

// After (v4)
[ClassCleanup]
public static void ClassCleanup(TestContext testContext) { }

If you previously used ClassCleanupBehavior.EndOfAssembly, move that cleanup logic to an [AssemblyCleanup] method instead.

3.4 TestContext.Properties type change

TestContext.Properties changed from IDictionary to IDictionary<string, object>. Update any Contains calls to ContainsKey:

// Before (v3)
testContext.Properties.Contains("key");

// After (v4)
testContext.Properties.ContainsKey("key");

3.5 TestTimeout enum removed

The TestTimeout enum (with only TestTimeout.Infinite) is removed. Replace with int.MaxValue:

// Before (v3)
[Timeout(TestTimeout.Infinite)]

// After (v4)
[Timeout(int.MaxValue)]

3.6 TestContext.ManagedType removed

The TestContext.ManagedType property is removed. Use TestContext.FullyQualifiedTestClassName instead.

3.7 Assert API signature changes

• Message + params removed: Assert methods that accepted both message and object[] parameters now accept only message. Use string interpolation instead of format strings:

// Before (v3)
Assert.AreEqual(expected, actual, "Expected {0} but got {1}", expected, actual);

// After (v4)
Assert.AreEqual(expected, actual, $"Expected {expected} but got {actual}");

• Assert.ThrowsException renamed: The Assert.ThrowsException APIs are renamed. Use Assert.ThrowsExactly (strict type match) or Assert.Throws (accepts derived exception types):

// Before (v3)
Assert.ThrowsException<InvalidOperationException>(() => DoSomething());

// After (v4) -- exact type match (same behavior as old ThrowsException)
Assert.ThrowsExactly<InvalidOperationException>(() => DoSomething());

// After (v4) -- also catches derived exception types
Assert.Throws<InvalidOperationException>(() => DoSomething());

• Assert.IsInstanceOfType out parameter changed: Assert.IsInstanceOfType<T>(x, out var t) changes to var t = Assert.IsInstanceOfType<T>(x):

// Before (v3)
Assert.IsInstanceOfType<MyType>(obj, out var typed);

// After (v4)
var typed = Assert.IsInstanceOfType<MyType>(obj);

Apply this assignment rewrite to every occurrence, preserving the concrete asserted type and all later uses of the typed variable. When source is available, show or edit the actual method rather than substituting a generic MyType example, then verify that the project compiles.

• Assert.AreEqual for IEquatable<T> removed: If you get generic type inference errors, explicitly specify the type argument as object.

3.8 ExpectedExceptionAttribute removed

The [ExpectedException] attribute is removed in v4. In MSTest 3.2, the MSTEST0006 analyzer was introduced to flag [ExpectedException] usage and suggest migrating to Assert.ThrowsExactly while still on v3 (a non-breaking change). In v4, the attribute is gone entirely. Migrate to Assert.ThrowsExactly:

// Before (v3)
[ExpectedException(typeof(InvalidOperationException))]
[TestMethod]
public void TestMethod()
{
    MyCall();
}

// After (v4)
[TestMethod]
public void TestMethod()
{
    Assert.ThrowsExactly<InvalidOperationException>(() => MyCall());
}

When the test has setup code before the throwing call, wrap only the throwing call in the lambda -- keep Arrange/Act separation clear:

// Before (v3)
[ExpectedException(typeof(ArgumentNullException))]
[TestMethod]
public void Validate_NullInput_Throws()
{
    var service = new ValidationService();
    service.Validate(null);  // throws here
}

// After (v4)
[TestMethod]
public void Validate_NullInput_Throws()
{
    var service = new ValidationService();
    Assert.ThrowsExactly<ArgumentNullException>(() => service.Validate(null));
}

For async test methods, use Assert.ThrowsExactlyAsync:

// Before (v3)
[ExpectedException(typeof(HttpRequestException))]
[TestMethod]
public async Task FetchData_BadUrl_Throws()
{
    await client.GetAsync("https://localhost:0");
}

// After (v4)
[TestMethod]
public async Task FetchData_BadUrl_Throws()
{
    await Assert.ThrowsExactlyAsync<HttpRequestException>(
        () => client.GetAsync("https://localhost:0"));
}

If [ExpectedException] used the AllowDerivedTypes property, use Assert.ThrowsAsync<T> (base type matching) instead of Assert.ThrowsExactlyAsync<T> (exact type matching).

For a focused migration, convert every attributed method in the supplied source, wrap only the statement expected to throw, preserve arrange/setup statements outside the lambda, and run the affected tests. A prose-only API substitution is incomplete when editable project files are present.

3.9 Dropped target frameworks

MSTest v4 supports .NET 8 and later and .NET Framework 4.6.2 and later. Platform-specific supported targets also include uap10.0.16299 (UWP), with modern UWP and WinUI using their corresponding supported Windows-specific .NET TFMs. .NET Core 3.1 through .NET 7 are dropped.

If the test project targets an unsupported framework, update TargetFramework:

<!-- Before -->
<TargetFramework>net6.0</TargetFramework>

<!-- After -->
<TargetFramework>net8.0</TargetFramework>

3.10 Unfolding strategy moved to TestMethodAttribute

The UnfoldingStrategy property (introduced in MSTest 3.7) has moved from individual data source attributes (DataRowAttribute, DynamicDataAttribute) to TestMethodAttribute.

3.11 ConditionBaseAttribute.ShouldRun renamed

The ConditionBaseAttribute.ShouldRun property is renamed to IsConditionMet.

3.12 Internal/removed types

Several types previously public are now internal or removed:

• MSTestDiscoverer, MSTestExecutor, AssemblyResolver, LogMessageListener
• TestExecutionManager, TestMethodInfo, TestResultExtensions
• UnitTestOutcomeExtensions, GenericParameterHelper
• ITestMethod in PlatformServices assembly (the one in TestFramework is unchanged)

If your code references any of these, find alternative approaches or remove the dependency.

Step 4: Address behavioral changes

These changes won't cause build errors but may affect test runtime behavior.

4.1 DisableAppDomain defaults to true

AppDomains are disabled by default. On .NET Framework, when running inside testhost (the default for dotnet test and VS), MSTest re-enables AppDomains automatically. If you need to explicitly control AppDomain isolation, set it via .runsettings:

<RunSettings>
  <RunConfiguration>
    <DisableAppDomain>false</DisableAppDomain>
  </RunConfiguration>
</RunSettings>

4.2 TestContext throws when used incorrectly

MSTest v4 now throws when accessing test-specific properties in the wrong lifecycle stage:

• TestContext.FullyQualifiedTestClassName -- cannot be accessed in [AssemblyInitialize]
• TestContext.TestName -- cannot be accessed in [AssemblyInitialize] or [ClassInitialize]

Fix: Move any code that accesses TestContext.TestName from [ClassInitialize] to [TestInitialize] or individual test methods, where per-test context is available. Do not replace TestName with FullyQualifiedTestClassName as a workaround -- they have different semantics.

4.3 TestCase.Id generation changed

The generation algorithm for TestCase.Id has changed to fix long-standing bugs. This may affect Azure DevOps test result tracking (e.g., test failure tracking over time). There is no code fix needed, but be aware of test result history discontinuity.

4.4 TreatDiscoveryWarningsAsErrors defaults to true

v4 uses stricter defaults. Discovery warnings are now treated as errors, which means tests that previously ran despite discovery issues may now fail entirely. If you see unexpected test failures after upgrading (not build errors, but tests not being discovered), check for discovery warnings. To restore v3 behavior while you investigate:

<RunSettings>
  <MSTest>
    <TreatDiscoveryWarningsAsErrors>false</TreatDiscoveryWarningsAsErrors>
  </MSTest>
</RunSettings>

Recommended: Fix the underlying discovery warnings rather than suppressing this setting.

4.5 MSTest.Sdk and vstest.console compatibility

MSTest.Sdk defaults to Microsoft.Testing.Platform (MTP) mode. MSTest.Sdk v3 still added Microsoft.NET.Test.Sdk in that mode; v4 removes the unnecessary reference. A CI pipeline that separately invokes vstest.console can therefore drop to zero discovered tests immediately after the v4 upgrade.

Option A -- Preserve MTP and transitional VSTest discovery: Add the exact compatible Microsoft.NET.Test.Sdk package explicitly. This is the least disruptive fix when MTP remains the primary runner but an existing vstest.console job cannot be removed yet:

Use a direct PackageReference with the exact compatible version resolved from the configured feed. Under Central Package Management, add or update the Microsoft.NET.Test.Sdk PackageVersion in Directory.Packages.props and keep the project reference versionless. Do not copy a fixed example version.

Verify with the actual vstest.console command; a passing dotnet test MTP run does not prove VSTest discovery.

Option B -- Switch the project to VSTest mode: Set the UseVSTest property. MSTest.Sdk then adds Microsoft.NET.Test.Sdk:

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

Keep the resolved exact MSTest.Sdk v4 pin from Step 2; this option changes the runner, not the selected MSTest version or target framework.

Option C -- Switch CI to dotnet test: Replace vstest.console invocations in your CI pipeline with dotnet test. This works natively with MTP and is the recommended long-term approach for MSTest.Sdk projects.

Do not say this behavior predates v4: removal of the transitive Microsoft.NET.Test.Sdk reference in MTP mode is one of the v4 behavioral breaking changes.

4.6 Analyzer severity changes

Multiple analyzers have been upgraded from Info to Warning by default:

• MSTEST0001, MSTEST0007, MSTEST0017, MSTEST0023, MSTEST0024, MSTEST0025
• MSTEST0030, MSTEST0031, MSTEST0032, MSTEST0035, MSTEST0037, MSTEST0045

Review and fix any new warnings, or suppress them in .editorconfig if intentional.

Step 5: Verify

1. Run dotnet build -- confirm zero errors and review any new warnings
2. Run dotnet test -- confirm all tests pass
3. Compare test results (pass/fail counts) to the pre-migration baseline
4. If using Azure DevOps test tracking, be aware that TestCase.Id changes may affect history continuity
5. Check that no tests were silently dropped due to stricter discovery

Validation

• All MSTest packages updated to 4.x
• Project builds with zero errors
• All tests pass with dotnet test
• Custom TestMethodAttribute subclasses updated for ExecuteAsync and CallerInfo
• ExpectedExceptionAttribute replaced with Assert.ThrowsExactly
• Assert.ThrowsException replaced with Assert.ThrowsExactly (or Assert.Throws)
• ClassCleanupBehavior enum usages removed
• TestContext.Properties.Contains updated to ContainsKey
• All target frameworks are net8.0+, net9.0, net462+, uap10.0.16299, or WinUI
• Behavioral changes reviewed and addressed
• No tests were lost during migration (compare test counts)

Related Skills

• writing-mstest-tests -- for modern MSTest v4 assertion APIs and test authoring best practices
• run-tests -- for running tests after migration

Common Pitfalls