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name: testing-patterns description: Unit testing patterns with xUnit, Moq, and FluentAssertions including AAA pattern, naming conventions, mocking, and edge case coverage. Use when writing or reviewing unit tests.

Testing Patterns

Overview

Tests ensure code correctness and enable confident refactoring. Follow these patterns for consistent, maintainable tests.

Test Framework Stack

• xUnit - Test framework
• Moq - Mocking library
• FluentAssertions - Readable assertions

Test Structure

Arrange-Act-Assert (AAA)

Every test follows this pattern:

Copy[Fact]
public async Task GetByIdAsync_WithValidId_ReturnsTask()
{
    // Arrange
    var taskId = Guid.NewGuid();
    var expectedTask = new TaskItem { Id = taskId, Title = "Test Task" };
    _repositoryMock.Setup(r => r.GetByIdAsync(taskId, It.IsAny<CancellationToken>()))
        .ReturnsAsync(expectedTask);

    // Act
    var result = await _sut.GetByIdAsync(taskId);

    // Assert
    result.Should().NotBeNull();
    result!.Id.Should().Be(taskId);
    result.Title.Should().Be("Test Task");
}

Test Class Structure

Copypublic class TaskServiceTests
{
    private readonly Mock<ITaskRepository> _repositoryMock;
    private readonly Mock<ILogger<TaskService>> _loggerMock;
    private readonly TaskService _sut; // System Under Test

    public TaskServiceTests()
    {
        _repositoryMock = new Mock<ITaskRepository>();
        _loggerMock = new Mock<ILogger<TaskService>>();
        _sut = new TaskService(_repositoryMock.Object, _loggerMock.Object);
    }

    // Tests...
}

Naming Conventions

Test Method Names

Format: MethodName_Scenario_ExpectedResult

Copy// Good - descriptive names
public async Task GetByIdAsync_WithValidId_ReturnsTask()
public async Task GetByIdAsync_WithNonExistentId_ReturnsNull()
public async Task CreateAsync_WithValidRequest_CreatesAndReturnsTask()
public async Task DeleteAsync_WithExistingId_ReturnsTrue()

// Bad - unclear names
public async Task TestGet()
public async Task Test1()

Mocking with Moq

Setup Mock Returns

Copy// Return a value
_repositoryMock.Setup(r => r.GetByIdAsync(taskId, It.IsAny<CancellationToken>()))
    .ReturnsAsync(expectedTask);

// Return null
_repositoryMock.Setup(r => r.GetByIdAsync(It.IsAny<Guid>(), It.IsAny<CancellationToken>()))
    .ReturnsAsync((TaskItem?)null);

Verify Mock Calls

Copy_repositoryMock.Verify(r => r.CreateAsync(It.IsAny<TaskItem>(), It.IsAny<CancellationToken>()), Times.Once);
_repositoryMock.Verify(r => r.DeleteAsync(taskId, It.IsAny<CancellationToken>()), Times.Once);

FluentAssertions

Basic Assertions

Copyresult.Should().Be(expected);
result.Should().BeNull();
result.Should().NotBeNull();
items.Should().HaveCount(3);
items.Should().BeEmpty();
result.IsCompleted.Should().BeTrue();

Edge Case Coverage

Always Test These Scenarios

• GET: found, not found, empty list
• CREATE: success, validates input, sets defaults
• UPDATE: success, not found, partial update
• DELETE: success, not found