Testing in .NET

// install: dotnet add package xunit; dotnet add package Moq; dotnet add package FluentAssertions

// Service under test
public class OrderService {
    private readonly IOrderRepository _repo;
    private readonly IEmailService _email;
    public OrderService(IOrderRepository repo, IEmailService email) {
        _repo = repo; _email = email;
    }
    public async Task<Order> CreateOrder(CreateOrderDto dto) {
        var order = new Order { CustomerId = dto.CustomerId, Total = dto.Total };
        await _repo.AddAsync(order);
        await _email.SendOrderConfirmation(dto.CustomerId, order.Id);
        return order;
    }
}

// Unit test with xUnit + Moq
public class OrderServiceTests {
    private readonly Mock<IOrderRepository> _repoMock = new();
    private readonly Mock<IEmailService> _emailMock = new();
    private readonly OrderService _sut;

    public OrderServiceTests() =>
        _sut = new OrderService(_repoMock.Object, _emailMock.Object);

    [Fact]
    public async Task CreateOrder_ShouldSaveAndSendEmail() {
        // Arrange
        var dto = new CreateOrderDto { CustomerId = 1, Total = 99.99m };
        _repoMock.Setup(r => r.AddAsync(It.IsAny<Order>())).Returns(Task.CompletedTask);

        // Act
        var result = await _sut.CreateOrder(dto);

        // Assert (FluentAssertions)
        result.Should().NotBeNull();
        result.Total.Should().Be(99.99m);
        _repoMock.Verify(r => r.AddAsync(It.IsAny<Order>()), Times.Once);
        _emailMock.Verify(e => e.SendOrderConfirmation(1, It.IsAny<int>()), Times.Once);
    }

    [Theory]
    [InlineData(-1)]
    [InlineData(0)]
    public async Task CreateOrder_NegativeTotal_ShouldThrow(decimal total) {
        var dto = new CreateOrderDto { CustomerId = 1, Total = total };
        await _sut.Invoking(s => s.CreateOrder(dto))
            .Should().ThrowAsync<ArgumentException>()
            .WithMessage("*total*");
    }
}

// Integration test with WebApplicationFactory
public class OrdersApiTests : IClassFixture<WebApplicationFactory<Program>> {
    private readonly HttpClient _client;
    public OrdersApiTests(WebApplicationFactory<Program> factory) =>
        _client = factory.WithWebHostBuilder(b =>
            b.ConfigureTestServices(s => s.AddSqlite("Data Source=:memory:")))
        .CreateClient();

    [Fact]
    public async Task PostOrder_ReturnsCreated() {
        var dto = new { customerId = 1, total = 49.99 };
        var response = await _client.PostAsJsonAsync("/api/orders", dto);
        response.StatusCode.Should().Be(HttpStatusCode.Created);
    }
}