csharp-mstest

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# 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`

```csharp
[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

```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());

From viniciuscs84/sdd-toolkit, MIT.

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