Csharp Mstest Agent Skill

by github

Get best practices for MSTest 3.x/4.x unit testing, including modern assertion APIs and data-driven tests

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the whole folder, loaded on every use

files

instructions only

copies elsewhere

how many repositories repackaged it

37394

stars on the repo

on the repository, not the skill itself

Install

one command, takes just this skill from the repository

npx skills add https://github.com/github/awesome-copilot --skill csharp-mstest

The instruction itself

31 sections, as written by the author

MSTest Best Practices (MSTest 3.x/4.x)

Your goal is to help me write effective unit tests with modern MSTest, using current APIs and best practices.

Project Setup

Use a separate test project with naming convention [ProjectName].Tests
Reference MSTest 3.x+ NuGet packages (includes analyzers)
Consider using MSTest.Sdk for simplified project setup
Run tests with dotnet test

Test Class Structure

Use [TestClass] attribute for test classes
Seal test classes by default for performance and design clarity
Use [TestMethod] for test methods (prefer over [DataTestMethod])
Follow Arrange-Act-Assert (AAA) pattern
Name tests using pattern MethodName_Scenario_ExpectedBehavior

[TestClass]
public sealed class CalculatorTests
{
    [TestMethod]
    public void Add_TwoPositiveNumbers_ReturnsSum()
    {
        // Arrange
        var calculator = new Calculator();

        // Act
        var result = calculator.Add(2, 3);

        // Assert
        Assert.AreEqual(5, result);
    }
}

Test Lifecycle

Prefer constructors over [TestInitialize] - enables readonly fields and follows standard C# patterns
Use [TestCleanup] for cleanup that must run even if test fails
Combine constructor with async [TestInitialize] when async setup is needed

[TestClass]
public sealed class ServiceTests
{
    private readonly MyService _service;  // readonly enabled by constructor

    public ServiceTests()
    {
        _service = new MyService();
    }

    [TestInitialize]
    public async Task InitAsync()
    {
        // Use for async initialization only
        await _service.WarmupAsync();
    }

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

Execution Order

Assembly Initialization - [AssemblyInitialize] (once per test assembly)
Class Initialization - [ClassInitialize] (once per test class)
Test Initialization (for every test method):
Constructor
Set TestContext property
[TestInitialize]
Test Execution - test method runs
Test Cleanup (for every test method):
[TestCleanup]
DisposeAsync (if implemented)
Dispose (if implemented)
Class Cleanup - [ClassCleanup] (once per test class)
Assembly Cleanup - [AssemblyCleanup] (once per test assembly)

Modern Assertion APIs

MSTest provides three assertion classes: Assert, StringAssert, and CollectionAssert.

Assert Class - Core Assertions

// Equality
Assert.AreEqual(expected, actual);
Assert.AreNotEqual(notExpected, actual);
Assert.AreSame(expectedObject, actualObject);      // Reference equality
Assert.AreNotSame(notExpectedObject, actualObject);

// Null checks
Assert.IsNull(value);
Assert.IsNotNull(value);

// Boolean
Assert.IsTrue(condition);
Assert.IsFalse(condition);

// Fail/Inconclusive
Assert.Fail("Test failed due to...");
Assert.Inconclusive("Test cannot be completed because...");

Exception Testing (Prefer over `[ExpectedException]`)

// Assert.Throws - matches TException or derived types
var ex = Assert.Throws<ArgumentException>(() => Method(null));
Assert.AreEqual("Value cannot be null.", ex.Message);

// Assert.ThrowsExactly - matches exact type only
var ex = Assert.ThrowsExactly<InvalidOperationException>(() => Method());

// Async versions
var ex = await Assert.ThrowsAsync<HttpRequestException>(async () => await client.GetAsync(url));
var ex = await Assert.ThrowsExactlyAsync<InvalidOperationException>(async () => await Method());

Collection Assertions (Assert class)

Assert.Contains(expectedItem, collection);
Assert.DoesNotContain(unexpectedItem, collection);
Assert.ContainsSingle(collection);  // exactly one element
Assert.HasCount(5, collection);
Assert.IsEmpty(collection);
Assert.IsNotEmpty(collection);

String Assertions (Assert class)

Assert.Contains("expected", actualString);
Assert.StartsWith("prefix", actualString);
Assert.EndsWith("suffix", actualString);
Assert.DoesNotStartWith("prefix", actualString);
Assert.DoesNotEndWith("suffix", actualString);
Assert.MatchesRegex(@"\d{3}-\d{4}", phoneNumber);
Assert.DoesNotMatchRegex(@"\d+", textOnly);

Comparison Assertions

Assert.IsGreaterThan(lowerBound, actual);
Assert.IsGreaterThanOrEqualTo(lowerBound, actual);
Assert.IsLessThan(upperBound, actual);
Assert.IsLessThanOrEqualTo(upperBound, actual);
Assert.IsInRange(actual, low, high);
Assert.IsPositive(number);
Assert.IsNegative(number);

Type Assertions

// MSTest 3.x - uses out parameter
Assert.IsInstanceOfType<MyClass>(obj, out var typed);
typed.DoSomething();

// MSTest 4.x - returns typed result directly
var typed = Assert.IsInstanceOfType<MyClass>(obj);
typed.DoSomething();

Assert.IsNotInstanceOfType<WrongType>(obj);

Assert.That (MSTest 4.0+)

Assert.That(result.Count > 0);  // Auto-captures expression in failure message

StringAssert Class

> Note: Prefer Assert class equivalents when available (e.g., Assert.Contains("expected", actual) over StringAssert.Contains(actual, "expected")).

StringAssert.Contains(actualString, "expected");
StringAssert.StartsWith(actualString, "prefix");
StringAssert.EndsWith(actualString, "suffix");
StringAssert.Matches(actualString, new Regex(@"\d{3}-\d{4}"));
StringAssert.DoesNotMatch(actualString, new Regex(@"\d+"));

CollectionAssert Class

> Note: Prefer Assert class equivalents when available (e.g., Assert.Contains).

// Containment
CollectionAssert.Contains(collection, expectedItem);
CollectionAssert.DoesNotContain(collection, unexpectedItem);

// Equality (same elements, same order)
CollectionAssert.AreEqual(expectedCollection, actualCollection);
CollectionAssert.AreNotEqual(unexpectedCollection, actualCollection);

// Equivalence (same elements, any order)
CollectionAssert.AreEquivalent(expectedCollection, actualCollection);
CollectionAssert.AreNotEquivalent(unexpectedCollection, actualCollection);

// Subset checks
CollectionAssert.IsSubsetOf(subset, superset);
CollectionAssert.IsNotSubsetOf(notSubset, collection);

// Element validation
CollectionAssert.AllItemsAreInstancesOfType(collection, typeof(MyClass));
CollectionAssert.AllItemsAreNotNull(collection);
CollectionAssert.AllItemsAreUnique(collection);

Data-Driven Tests

DataRow

[TestMethod]
[DataRow(1, 2, 3)]
[DataRow(0, 0, 0, DisplayName = "Zeros")]
[DataRow(-1, 1, 0, IgnoreMessage = "Known issue #123")]  // MSTest 3.8+
public void Add_ReturnsSum(int a, int b, int expected)
{
    Assert.AreEqual(expected, Calculator.Add(a, b));
}

DynamicData

The data source can return any of the following types:

IEnumerable<(T1, T2, ...)> (ValueTuple) - preferred, provides type safety (MSTest 3.7+)
IEnumerable<Tuple<T1, T2, ...>> - provides type safety
IEnumerable<TestDataRow> - provides type safety plus control over test metadata (display name, categories)
IEnumerable<object[]> - least preferred, no type safety

> Note: When creating new test data methods, prefer ValueTuple or TestDataRow over IEnumerable<object[]>. The object[] approach provides no compile-time type checking and can lead to runtime errors from type mismatches.

[TestMethod]
[DynamicData(nameof(TestData))]
public void DynamicTest(int a, int b, int expected)
{
    Assert.AreEqual(expected, Calculator.Add(a, b));
}

// ValueTuple - preferred (MSTest 3.7+)
public static IEnumerable<(int a, int b, int expected)> TestData =>
[
    (1, 2, 3),
    (0, 0, 0),
];

// TestDataRow - when you need custom display names or metadata
public static IEnumerable<TestDataRow<(int a, int b, int expected)>> TestDataWithMetadata =>
[
    new((1, 2, 3)) { DisplayName = "Positive numbers" },
    new((0, 0, 0)) { DisplayName = "Zeros" },
    new((-1, 1, 0)) { DisplayName = "Mixed signs", IgnoreMessage = "Known issue #123" },
];

// IEnumerable<object[]> - avoid for new code (no type safety)
public static IEnumerable<object[]> LegacyTestData =>
[
    [1, 2, 3],
    [0, 0, 0],
];

TestContext

The TestContext class provides test run information, cancellation support, and output methods.

See TestContext documentation for complete reference.

Accessing TestContext

// Property (MSTest suppresses CS8618 - don't use nullable or = null!)
public TestContext TestContext { get; set; }

// Constructor injection (MSTest 3.6+) - preferred for immutability
[TestClass]
public sealed class MyTests
{
    private readonly TestContext _testContext;

    public MyTests(TestContext testContext)
    {
        _testContext = testContext;
    }
}

// Static methods receive it as parameter
[ClassInitialize]
public static void ClassInit(TestContext context) { }

// Optional for cleanup methods (MSTest 3.6+)
[ClassCleanup]
public static void ClassCleanup(TestContext context) { }

[AssemblyCleanup]
public static void AssemblyCleanup(TestContext context) { }

Cancellation Token

Always use TestContext.CancellationToken for cooperative cancellation with [Timeout]:

[TestMethod]
[Timeout(5000)]
public async Task LongRunningTest()
{
    await _httpClient.GetAsync(url, TestContext.CancellationToken);
}

Test Run Properties

TestContext.TestName              // Current test method name
TestContext.TestDisplayName       // Display name (3.7+)
TestContext.CurrentTestOutcome    // Pass/Fail/InProgress
TestContext.TestData              // Parameterized test data (3.7+, in TestInitialize/Cleanup)
TestContext.TestException         // Exception if test failed (3.7+, in TestCleanup)
TestContext.DeploymentDirectory   // Directory with deployment items

Output and Result Files

// Write to test output (useful for debugging)
TestContext.WriteLine("Processing item {0}", itemId);

// Attach files to test results (logs, screenshots)
TestContext.AddResultFile(screenshotPath);

// Store/retrieve data across test methods
TestContext.Properties["SharedKey"] = computedValue;

Advanced Features

Retry for Flaky Tests (MSTest 3.9+)

[TestMethod]
[Retry(3)]
public void FlakyTest() { }

Conditional Execution (MSTest 3.10+)

Skip or run tests based on OS or CI environment:

// OS-specific tests
[TestMethod]
[OSCondition(OperatingSystems.Windows)]
public void WindowsOnlyTest() { }

[TestMethod]
[OSCondition(OperatingSystems.Linux | OperatingSystems.MacOS)]
public void UnixOnlyTest() { }

[TestMethod]
[OSCondition(ConditionMode.Exclude, OperatingSystems.Windows)]
public void SkipOnWindowsTest() { }

// CI environment tests
[TestMethod]
[CICondition]  // Runs only in CI (default: ConditionMode.Include)
public void CIOnlyTest() { }

[TestMethod]
[CICondition(ConditionMode.Exclude)]  // Skips in CI, runs locally
public void LocalOnlyTest() { }

Parallelization

// Assembly level
[assembly: Parallelize(Workers = 4, Scope = ExecutionScope.MethodLevel)]

// Disable for specific class
[TestClass]
[DoNotParallelize]
public sealed class SequentialTests { }

Work Item Traceability (MSTest 3.8+)

Link tests to work items for traceability in test reports:

// Azure DevOps work items
[TestMethod]
[WorkItem(12345)]  // Links to work item #12345
public void Feature_Scenario_ExpectedBehavior() { }

// Multiple work items
[TestMethod]
[WorkItem(12345)]
[WorkItem(67890)]
public void Feature_CoversMultipleRequirements() { }

// GitHub issues (MSTest 3.8+)
[TestMethod]
[GitHubWorkItem("https://github.com/owner/repo/issues/42")]
public void BugFix_Issue42_IsResolved() { }

Work item associations appear in test results and can be used for:

Tracing test coverage to requirements
Linking bug fixes to regression tests
Generating traceability reports in CI/CD pipelines

Common Mistakes to Avoid

// ❌ Wrong argument order
Assert.AreEqual(actual, expected);
// ✅ Correct
Assert.AreEqual(expected, actual);

// ❌ Using ExpectedException (obsolete)
[ExpectedException(typeof(ArgumentException))]
// ✅ Use Assert.Throws
Assert.Throws<ArgumentException>(() => Method());

// ❌ Using LINQ Single() - unclear exception
var item = items.Single();
// ✅ Use ContainsSingle - better failure message
var item = Assert.ContainsSingle(items);

// ❌ Hard cast - unclear exception
var handler = (MyHandler)result;
// ✅ Type assertion - shows actual type on failure
var handler = Assert.IsInstanceOfType<MyHandler>(result);

// ❌ Ignoring cancellation token
await client.GetAsync(url, CancellationToken.None);
// ✅ Flow test cancellation
await client.GetAsync(url, TestContext.CancellationToken);

// ❌ Making TestContext nullable - leads to unnecessary null checks
public TestContext? TestContext { get; set; }
// ❌ Using null! - MSTest already suppresses CS8618 for this property
public TestContext TestContext { get; set; } = null!;
// ✅ Declare without nullable or initializer - MSTest handles the warning
public TestContext TestContext { get; set; }

Test Organization

Group tests by feature or component
Use [TestCategory("Category")] for filtering
Use [TestProperty("Name", "Value")] for custom metadata (e.g., [TestProperty("Bug", "12345")])
Use [Priority(1)] for critical tests
Enable relevant MSTest analyzers (MSTEST0020 for constructor preference)

Mocking and Isolation

Use Moq or NSubstitute for mocking dependencies
Use interfaces to facilitate mocking
Mock dependencies to isolate units under test

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How to use it

Copy the folder

Take github/csharp-mstest from the repository into ~/.claude/skills for personal use, or into .claude/skills inside a project.

Check the name does not clash

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Csharp Mstest Agent Skill

Install

The instruction itself

MSTest Best Practices (MSTest 3.x/4.x)

Project Setup

Test Class Structure

Test Lifecycle

Execution Order

Modern Assertion APIs

Assert Class - Core Assertions

Exception Testing (Prefer over [ExpectedException])

Collection Assertions (Assert class)

String Assertions (Assert class)

Comparison Assertions

Type Assertions

Assert.That (MSTest 4.0+)

StringAssert Class

CollectionAssert Class

Data-Driven Tests

DataRow

DynamicData

TestContext

Accessing TestContext

Cancellation Token

Test Run Properties

Output and Result Files

Advanced Features

Retry for Flaky Tests (MSTest 3.9+)

Conditional Execution (MSTest 3.10+)

Parallelization

Work Item Traceability (MSTest 3.8+)

Common Mistakes to Avoid

Test Organization

Mocking and Isolation

Other skills for the same job

How to use it

Exception Testing (Prefer over `[ExpectedException]`)