---

name: writing-mstest-tests description: > ALWAYS USE when asked to fix, rewrite, update, improve, modernize, show corrected code for, or explain existing MSTest tests or MSTest-specific configuration. Use for "review" when corrected code or edits are wanted, even for one pasted assertion or passing tests with bad failure output. Covers expected/actual labels; generic Boolean, collection, string, numeric, null, identity, exception, hard-cast, and object[] checks; TestContext/lifecycle; timeout/cancellation; OS/CI conditions, retry, cleanup, parallelization, MSTest.Sdk project setup, and MSTESTxxxx. Honor the installed MSTest version. DO NOT USE to design new test cases (code-testing-agent), perform report-only audits, create project files rather than explain MSTest setup, run tests, migrate frameworks, or handle non-MSTest/non-.NET code. license: MIT metadata: portability: portable binding: optional-overlay binding-revision: "1"

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

| Input | Required | Description | |-------|----------|-------------| | Code under test | No | The production code to be tested | | Existing test code | No | Current tests to fix, update, or modernize | | Test scenario description | No | What behavior the user wants to test |

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

| API/pattern | Minimum version | Compatible fallback | |---|---:|---| | Assert.ThrowsExactly*, unified Assert.Contains / HasCount / IsEmpty / IsNotEmpty | 3.8 | Assert.ThrowsException*, CollectionAssert, StringAssert | | Assert.IsGreaterThan, IsLessThan, IsInRange, StartsWith, EndsWith, MatchesRegex | 3.10 | Assert.IsTrue with a clear message, or StringAssert | | Generic Assert.IsInstanceOfType<T>(value, out var typed) | 3.4-3.11 only | Non-generic assertion then post-assert cast on 3.0-3.3; v4 returns the typed value directly | | ValueTuple DynamicData | 3.7 | IEnumerable<object[]> | | Constructor injection of TestContext | 3.6 | Instance TestContext property | | [Retry], [OSCondition] | 3.8 | No built-in retry/OS condition; fix flakiness or retain the existing condition mechanism | | [CICondition] | 3.10 | Existing project-specific condition mechanism |

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.

| What you are testing | Assertion | |---|---| | Two values are equal | Assert.AreEqual(expected, actual) | | Same object instance (reference identity) | Assert.AreSame(expected, actual) | | Value is null | Assert.IsNull(value) | | Value is not null | Assert.IsNotNull(value) | | Collection is empty | Assert.IsEmpty(collection) (3.8+) or CollectionAssert / count assertion | | Collection is not empty | Assert.IsNotEmpty(collection) (3.8+) or count assertion | | Collection has exactly N items | Assert.HasCount(N, collection) (3.8+) or Assert.AreEqual on count | | Collection contains an item | Assert.Contains(item, collection) (3.8+) or CollectionAssert.Contains | | Collection does not contain an item | Assert.DoesNotContain(item, collection) (3.8+) or CollectionAssert.DoesNotContain | | Object is a specific type | Assert.IsInstanceOfType<T>(value) | | Code throws an exception | Assert.ThrowsExactly<T> (3.8+) or Assert.ThrowsException<T> (earlier) |

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(n