Improve best practices for MSTest

prompts/csharp-mstest.prompt.md

@@ -1,67 +1,475 @@
 ---
 agent: 'agent'
 tools: ['changes', 'search/codebase', 'edit/editFiles', 'problems', 'search']
-description: 'Get best practices for MSTest unit testing, including data-driven tests'
+description: 'Get best practices for MSTest 3.x/4.x unit testing, including modern assertion APIs and data-driven tests'
 ---
 
-# MSTest Best Practices
+# MSTest Best Practices (MSTest 3.x/4.x)
 
-Your goal is to help me write effective unit tests with MSTest, covering both standard and data-driven testing approaches.
+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 package
+- Reference MSTest 3.x+ NuGet packages (includes analyzers)
-- Create test classes that match the classes being tested (e.g., `CalculatorTests` for `Calculator`)
+- Consider using MSTest.Sdk for simplified project setup
-- Use .NET SDK test commands: `dotnet test` for running tests
+- Run tests with `dotnet test`
 
-## Test Structure
+## Test Class Structure
 
 - Use `[TestClass]` attribute for test classes
-- Use `[TestMethod]` attribute for test methods
+- **Seal test classes by default** for performance and design clarity
-- Follow the Arrange-Act-Assert (AAA) pattern
+- Use `[TestMethod]` for test methods (prefer over `[DataTestMethod]`)
-- Name tests using the pattern `MethodName_Scenario_ExpectedBehavior`
+- Follow Arrange-Act-Assert (AAA) pattern
-- Use `[TestInitialize]` and `[TestCleanup]` for per-test setup and teardown
+- Name tests using pattern `MethodName_Scenario_ExpectedBehavior`
-- Use `[ClassInitialize]` and `[ClassCleanup]` for per-class setup and teardown
-- Use `[AssemblyInitialize]` and `[AssemblyCleanup]` for assembly-level setup and teardown
 
-## Standard Tests
+```csharp
+[TestClass]
+public sealed class CalculatorTests
+{
+    [TestMethod]
+    public void Add_TwoPositiveNumbers_ReturnsSum()
+    {
+        // Arrange
+        var calculator = new Calculator();
 
-- Keep tests focused on a single behavior
+        // Act
-- Avoid testing multiple behaviors in one test method
+        var result = calculator.Add(2, 3);
-- Use clear assertions that express intent
+
-- Include only the assertions needed to verify the test case
+        // Assert
-- Make tests independent and idempotent (can run in any order)
+        Assert.AreEqual(5, result);
-- Avoid test interdependencies
+    }
+}
+```
+
+## 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
+
+```csharp
+[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
+
+1. **Assembly Initialization** - `[AssemblyInitialize]` (once per test assembly)
+2. **Class Initialization** - `[ClassInitialize]` (once per test class)
+3. **Test Initialization** (for every test method):
+   1. Constructor
+   2. Set `TestContext` property
+   3. `[TestInitialize]`
+4. **Test Execution** - test method runs
+5. **Test Cleanup** (for every test method):
+   1. `[TestCleanup]`
+   2. `DisposeAsync` (if implemented)
+   3. `Dispose` (if implemented)
+6. **Class Cleanup** - `[ClassCleanup]` (once per test class)
+7. **Assembly Cleanup** - `[AssemblyCleanup]` (once per test assembly)
+
+## Modern Assertion APIs
+
+MSTest provides three assertion classes: `Assert`, `StringAssert`, and `CollectionAssert`.
+
+### Assert Class - Core Assertions
+
+```csharp
+// 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]`)
+
+```csharp
+// 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)
+
+```csharp
+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)
+
+```csharp
+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
+
+```csharp
+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
+
+```csharp
+// Returns typed result for further assertions
+Assert.IsInstanceOfType<MyClass>(obj, out var typed);
+typed.DoSomething();
+
+Assert.IsNotInstanceOfType<WrongType>(obj);
+```
+
+### Assert.That (MSTest 4.0+)
+
+```csharp
+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")`).
+
+```csharp
+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`).
+
+```csharp
+// 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
 
-- Use `[TestMethod]` combined with data source attributes
+### DataRow
-- Use `[DataRow]` for inline test data
-- Use `[DynamicData]` for programmatically generated test data
-- Use `[TestProperty]` to add metadata to tests
-- Use meaningful parameter names in data-driven tests
 
-## Assertions
+```csharp
+[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));
+}
+```
 
-- Use `Assert.AreEqual` for value equality
+### DynamicData
-- Use `Assert.AreSame` for reference equality
-- Use `Assert.IsTrue`/`Assert.IsFalse` for boolean conditions
-- Use `CollectionAssert` for collection comparisons
-- Use `StringAssert` for string-specific assertions
-- Use `Assert.Throws<T>` to test exceptions
-- Ensure assertions are simple in nature and have a message provided for clarity on failure
 
-## Mocking and Isolation
+The data source can return any of the following types:
 
-- Consider using Moq or NSubstitute alongside MSTest
+- `IEnumerable<(T1, T2, ...)>` (ValueTuple) - **preferred**, provides type safety (MSTest 3.7+)
-- Mock dependencies to isolate units under test
+- `IEnumerable<Tuple<T1, T2, ...>>` - provides type safety
-- Use interfaces to facilitate mocking
+- `IEnumerable<TestDataRow>` - provides type safety plus control over test metadata (display name, categories)
-- Consider using a DI container for complex test setups
+- `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.
+
+```csharp
+[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](https://learn.microsoft.com/dotnet/core/testing/unit-testing-mstest-writing-tests-testcontext) for complete reference.
+
+### Accessing TestContext
+
+```csharp
+// 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]`:
+
+```csharp
+[TestMethod]
+[Timeout(5000)]
+public async Task LongRunningTest()
+{
+    await _httpClient.GetAsync(url, TestContext.CancellationToken);
+}
+```
+
+### Test Run Properties
+
+```csharp
+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
+
+```csharp
+// 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+)
+
+```csharp
+[TestMethod]
+[Retry(3)]
+public void FlakyTest() { }
+```
+
+### Conditional Execution (MSTest 3.10+)
+
+Skip or run tests based on OS or CI environment:
+
+```csharp
+// 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
+
+```csharp
+// 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:
+
+```csharp
+// 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
+
+```csharp
+// ❌ 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 test categories with `[TestCategory("Category")]`
+- Use `[TestCategory("Category")]` for filtering
-- Use test priorities with `[Priority(1)]` for critical tests
+- Use `[TestProperty("Name", "Value")]` for custom metadata (e.g., `[TestProperty("Bug", "12345")]`)
-- Use `[Owner("DeveloperName")]` to indicate ownership
+- 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