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

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

// 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)