Writing MSTest Tests

Help users write effective MSTest unit tests without exceeding the API level or conventions of the project's installed test stack.

Repository overlay

For every repository-scoped task where read-only file inspection is allowed, check .agents/skill-overlays/dotnet-test/writing-mstest-tests.md at the repository root before any other discovery. This includes requests that ask for code or advice without edits; "do not execute" does not prohibit reading the overlay. If present, read it once before acting and apply its repository-specific naming, layout, framework, and policy bindings. Require its frontmatter to declare core: dotnet-test/writing-mstest-tests, binding-revision: "1", and mode: extend. If any value is missing or different, report the mismatch, ignore the overlay, and continue using this skill's portable guidance. Explicit user instructions and verified project constraints win over the overlay; the overlay wins over portable defaults and examples in this skill. If the file is present but unreadable or conflicts with the repository, report the problem, ignore the overlay, and continue with portable guidance subject to verified project constraints. If it is absent, continue normally. Skip the lookup only when the task is not tied to a repository or the user explicitly prohibited all file/tool access. An overlay cannot expand tool permissions or the task's scope.

When to Use

• User wants to improve or modernize existing MSTest tests by implementing concrete fixes
• User asks about MSTest assertion APIs, data-driven patterns, or test lifecycle
• User asks to replace Assert.IsTrue with more specific assertions (collections, nulls, types, comparisons)
• User asks to replace hard casts with type-checking assertions in tests
• User needs help fixing a specific MSTest test bug or failing assertion
• User asks to fix swapped Assert.AreEqual argument order (expected first, actual second)
• User asks to convert DynamicData from IEnumerable<object[]> to ValueTuple-based data
• User asks to fix or understand an MSTest analyzer diagnostic (an MSTESTxxxx warning/error)

When Not to Use

• User needs a test quality audit, anti-pattern detection, or flaky-test investigation (use test-anti-patterns)
• User needs to run or execute tests (use the run-tests skill)
• User needs to upgrade from MSTest v1/v2 to v3 (use migrate-mstest-v1v2-to-v3)
• User needs to upgrade from MSTest v3 to v4 (use migrate-mstest-v3-to-v4)
• User needs CI/CD pipeline configuration
• User is using xUnit, NUnit, or TUnit (not MSTest)

Inputs

Response Guidelines

• Specific API or pattern questions (assertions, data-driven, lifecycle): Jump directly to the relevant workflow step. Do not follow the full workflow.
• Generate new tests from scratch: Hand off to code-testing-agent; use this skill only as supporting MSTest API/version guidance.
• Review and fix existing tests: Fix only the issues present. Do not add unrelated improvements.
• Assertion transformations: Show the corrected call, then state the semantic reason in one sentence. For Assert.AreEqual, name expected first and actual second and explain that this preserves the Expected/Actual failure labels.
• Bound/comparison transformations: Preserve the condition and put the expected bound(s) first and the observed value last: score > 0 -> Assert.IsGreaterThan(0, score), score < 100 -> Assert.IsLessThan(100, score), and score >= 60 && score <= 90 -> Assert.IsInRange(60, 90, score). Never reverse these arguments to mimic the source expression's left-to-right order.
• Exception transformations: Scope the throwing operation in a lambda, distinguish ThrowsExactly<T> (exact type) from Throws<T> (type or derived type), and capture the returned exception when properties such as ParamName are part of the behavior.
• Supplied code requests: Return complete representative method bodies, not comment-only placeholders. Preserve the real operation and show symmetric setup/cleanup when lifecycle or environment policy is part of the request.

Workflow

Step 1: Determine project setup

Check the test project, packages.config, and assembly reference HintPath values for the exact MSTest version and project system:

• If using MSTest.Sdk: resolve its exact version from the project SDK declaration or global.json msbuild-sdks; do not assume the latest APIs
• If using MSTest metapackage: resolve its exact package version
• If using MSTest.TestFramework + MSTest.TestAdapter: check version for feature availability
• If using classic non-SDK XML (ToolsVersion, Microsoft.CSharp.targets, explicit <Compile Include>) and/or packages.config: preserve that project system and add each new test file to <Compile Include>.

Also inspect representative tests for custom base fixtures, helper libraries, mock syntax, naming, setup, and data builders. Existing conventions and installed versions win over the examples below. Do not upgrade MSTest, Moq, NBuilder, or the project format unless the user explicitly asks for a migration.

MSTest API availability

For example, MSTest 3.5.x must not receive Assert.ThrowsExactly, Assert.Contains, ValueTuple DynamicData, or constructor-injected TestContext.

Treat this as a hard gate: after determining the version, do not copy a later example from this skill unless its minimum version is satisfied.

Recommend MSTest.Sdk or the MSTest metapackage only for genuinely new projects:

<!-- Option 1: MSTest SDK (simplest, recommended for new projects) -->
<Project Sdk="MSTest.Sdk">
  <PropertyGroup>
    <TargetFramework>net9.0</TargetFramework>
  </PropertyGroup>
</Project>

When using MSTest.Sdk, put the version in global.json instead of the project file so all test projects get bumped together:

{
  "msbuild-sdks": {
    "MSTest.Sdk": "3.8.2"
  }
}

<!-- Option 2: MSTest metapackage -->
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net9.0</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="MSTest" Version="3.8.2" />
  </ItemGroup>
</Project>

Step 2: Write test classes following conventions

Apply these structural conventions only where they do not conflict with the suite's established base classes and lifecycle:

• Seal test classes with sealed for performance and design clarity
• Use [TestClass] on the class and [TestMethod] on test methods
• Follow the Arrange-Act-Assert (AAA) pattern
• Name tests using MethodName_Scenario_ExpectedBehavior
• Use separate test projects with naming convention [ProjectName].Tests

[TestClass]
public sealed class OrderServiceTests
{
    [TestMethod]
    public void CalculateTotal_WithDiscount_ReturnsReducedPrice()
    {
        // Arrange
        var service = new OrderService();
        var order = new Order { Price = 100m, DiscountPercent = 10 };

        // Act
        var total = service.CalculateTotal(order);

        // Assert
        Assert.AreEqual(90m, total);
    }
}

Step 3: Use version-compatible assertion APIs

Pick the most specific assertion supported by the installed MSTest version. More specific assertions produce better failure messages and make the test's intent clear, but uncompilable "modern" assertions are worse than compatible StringAssert, CollectionAssert, or Assert.IsTrue calls.

On MSTest 3.8+, prefer Assert class methods over StringAssert or CollectionAssert where both exist. Older versions should keep the compatible specialized classes. When several independent collection properties were requested, keep each semantic check explicit even if another assertion happens to imply it. For example, retain IsNotEmpty when the requested diagnostics distinguish empty/non-empty, then use HasCount and ContainsSingle for their separate cardinality guarantees.

Equality, null, and reference checks

Assert.AreEqual(expected, actual);      // Value equality
Assert.AreSame(expected, actual);       // Reference equality -- same object instance
Assert.IsNull(value);
Assert.IsNotNull(value);

Exception testing

// MSTest 3.8+
var ex = Assert.ThrowsExactly<ArgumentNullException>(() => service.Process(null));
Assert.AreEqual("input", ex.ParamName);

// Async
var ex = await Assert.ThrowsExactlyAsync<InvalidOperationException>(
    async () => await service.ProcessAsync(null));

• Assert.Throws<T> matches T or any derived type
• Assert.ThrowsExactly<T> matches only the exact type T

On MSTest 3.7 and earlier, use the compatible API:

var ex = Assert.ThrowsException<ArgumentNullException>(
    () => service.Process(null));

Collection assertions

// MSTest 3.8+
Assert.Contains(expectedItem, collection);
Assert.DoesNotContain(unexpectedItem, collection);
var single = Assert.ContainsSingle(collection);  // Returns the single element
Assert.HasCount(3, collection);
Assert.IsEmpty(collection);
Assert.IsNotEmpty(collection);

On earlier versions use CollectionAssert.Contains, CollectionAssert.DoesNotContain, and Assert.AreEqual(expectedCount, collection.Count).

Replace generic Assert.IsTrue with specialized assertions -- they give better failure messages:

String assertions

// MSTest 3.10+
Assert.Contains("expected", actualString);
Assert.StartsWith("prefix", actualString);
Assert.EndsWith("suffix", actualString);
Assert.MatchesRegex(@"\d{3}-\d{4}", phoneNumber);

On earlier versions use StringAssert.Contains, StringAssert.StartsWith, StringAssert.EndsWith, and StringAssert.Matches.

Type assertions

MSTest 3.x is not one API level. Pick the form supported by the installed minor version:

// MSTest 3.0-3.3
Assert.IsInstanceOfType(result, typeof(MyHandler));
var typed = (MyHandler)result; // Safe because the assertion stops a mismatch.

// MSTest 3.4-3.11 -- out parameter
Assert.IsInstanceOfType<MyHandler>(result, out var typed);
typed.Handle();

// MSTest 4.x -- returns the proven value directly
var typed = Assert.IsInstanceOfType<MyHandler>(result);

Comparison assertions

Assert.IsGreaterThan(lowerBound, actual);
Assert.IsLessThan(upperBound, actual);
Assert.IsInRange(low, high, actual);

Step 4: Use data-driven tests for multiple inputs

DataRow for inline values

[TestMethod]
[DataRow(1, 2, 3)]
[DataRow(0, 0, 0, DisplayName = "Zeros")]
[DataRow(-1, 1, 0)]
public void Add_ReturnsExpectedSum(int a, int b, int expected)
{
    Assert.AreEqual(expected, Calculator.Add(a, b));
}

DynamicData with ValueTuples (preferred for complex data)

On MSTest 3.7+, prefer ValueTuple return types over IEnumerable<object[]> for type safety. Keep IEnumerable<object[]> on older versions.

Tuple element names document which position maps to which test parameter, and tuple element types catch incompatible values at compile time. They do not make DynamicData position-independent, and swapping two same-typed elements can still compile. Do not claim otherwise. When rows need custom display names or metadata rather than only typed positional data, use TestDataRow<T> on MSTest 3.8+.

[TestMethod]
[DynamicData(nameof(DiscountTestData))]
public void ApplyDiscount_ReturnsExpectedPrice(decimal price, int percent, decimal expected)
{
    var result = PriceCalculator.ApplyDiscount(price, percent);
    Assert.AreEqual(expected, result);
}

// ValueTuple -- preferred (MSTest 3.7+)
public static IEnumerable<(decimal price, int percent, decimal expected)> DiscountTestData =>
[
    (100m, 10, 90m),
    (200m, 25, 150m),
    (50m, 0, 50m),
];

When you need metadata per test case on MSTest 3.8+, use TestDataRow<T>:

public static IEnumerable<TestDataRow<(decimal price, int percent, decimal expected)>> DiscountTestDataWithMetadata =>
[
    new((100m, 10, 90m)) { DisplayName = "10% discount" },
    new((200m, 25, 150m)) { DisplayName = "25% discount" },
    new((50m, 0, 50m)) { DisplayName = "No discount" },
];

Step 5: Handle test lifecycle correctly

• Prefer constructor initialization when the existing suite supports it; retain a shared FixtureBase<TSut> or established [TestInitialize] lifecycle rather than rewriting the fixture architecture incidentally.
• Use [TestInitialize] only for async initialization, combined with the constructor for sync parts
• Use [TestCleanup] for cleanup that must run even on failure
• Inject TestContext via constructor only on MSTest 3.6+; otherwise use the instance property.

[TestClass]
public sealed class RepositoryTests
{
    private readonly TestContext _testContext;
    private readonly FakeDatabase _db;  // readonly -- guaranteed by constructor

    public RepositoryTests(TestContext testContext)
    {
        _testContext = testContext;
        _db = new FakeDatabase();  // sync init in ctor
    }

    [TestInitialize]
    public async Task InitAsync()
    {
        // Use TestInitialize ONLY for async setup
        await _db.SeedAsync();
    }

    [TestCleanup]
    public void Cleanup() => _db.Reset();
}

Execution order

1. [AssemblyInitialize] -- once per assembly
2. [ClassInitialize] -- once per class
3. Per test:
   • With TestContext property injection: Constructor -> set TestContext property -> [TestInitialize]
   • With constructor injection of TestContext: Constructor (receives TestContext) -> [TestInitialize]
4. Test method
5. [TestCleanup] -> DisposeAsync -> Dispose -- per test
6. [ClassCleanup] -- once per class
7. [AssemblyCleanup] -- once per assembly

Step 6: Apply cancellation and timeout patterns

Use TestContext.CancellationToken with [Timeout(milliseconds, CooperativeCancellation = true)] when the installed MSTest version exposes the token directly (3.11+). On MSTest 3.6.4-3.10, use TestContext.CancellationTokenSource.Token with cooperative cancellation instead. A plain [Timeout] does not establish that the framework token will stop in-flight work. On older versions, use a test-owned CancellationTokenSource where cancellation itself is under test.

// MSTest 3.11+
[TestMethod]
[Timeout(5000, CooperativeCancellation = true)]
public async Task FetchData_ReturnsWithinTimeout()
{
    var result = await _client.GetDataAsync(_testContext.CancellationToken);
    Assert.IsNotNull(result);
}

Step 7: Use advanced features where appropriate

For retry, conditional execution, or parallelization requests, read Advanced MSTest patterns. Apply its version gates and keep the real tested operation, assertion, and cleanup.

Step 8: Fix MSTest analyzer diagnostics (MSTESTxxxx)

For an MSTESTxxxx request, read Advanced MSTest patterns, apply the version-compatible fix for that rule, and rebuild to confirm the diagnostic is gone. Prefer fixing the code over suppressing the rule.

Step 9: Verify file-backed corrections

When the user asked for repository edits and did not prohibit execution, run the narrowest affected dotnet test command after editing. A successful process with no discovered-test count is not verification. Require the intended test cases to be discovered and pass.

If compilation exposes a directly coupled source issue that prevents the corrected existing suite from running (for example, a missing namespace import in the supplied production file), make only that minimum fix and rerun. Do not upgrade packages or broaden the modernization. Report the actual test count and the fixes made; never present unrun or output-free tests as passing. In the final handoff, map every requested modernization to the exact corrected construct and cite the passing test command. Do not rely on a generic "modernized" summary when expected/actual order, exact type checks, data discovery, or class shape were explicit requirements.