Unit Testing ASP.NET Core with xUnit and Moq

Introduction

ShopNest Order Service logic — discounts, stock checks, totals — belongs in unit tests. One hour writing tests saves ten hours debugging production orders at 2 AM. This lesson uses xUnit and Moq with Arrange-Act-Assert on real order scenarios.

After this article you will

• Write [Fact] and [Theory] tests with xUnit
• Mock dependencies with Moq Setup and Verify
• Test services, validators, and controllers in isolation
• Follow naming conventions and AAA pattern
• Understand code coverage basics and TDD intro

Prerequisites

• Article 53 — gRPC with ASP.NET Core
• ShopNest Order Service / API from prior modules

Concept deep-dive

Why unit test?

ROI: catch regressions before deploy; document expected behavior; enable fearless refactoring. Indian service companies increasingly expect xUnit in interviews.

// Naming: MethodName_Scenario_ExpectedBehavior
public class OrderServiceTests
{
    [Fact]
    public async Task CreateOrderAsync_EmptyCart_ThrowsValidationException()
    {
        // Arrange
        var mockRepo = new Mock<IOrderRepository>();
        var svc = new OrderService(mockRepo.Object, Mock.Of<ILogger<OrderService>>());

        // Act & Assert
        await Assert.ThrowsAsync<ValidationException>(() =>
            svc.CreateOrderAsync(new CreateOrderDto { Lines = new() }, "user1"));
        mockRepo.Verify(r => r.AddAsync(It.IsAny<Order>(), default), Times.Never);
    }

    [Theory]
    [InlineData(0, false)]
    [InlineData(1, true)]
    [InlineData(100, true)]
    public void IsValidQuantity_ReturnsExpected(int qty, bool expected)
    {
        Assert.Equal(expected, OrderRules.IsValidQuantity(qty));
    }
}

Moq: Setup defines behavior; Verify asserts calls; ReturnsAsync for async methods.

Hands-on — ShopNest E-Commerce Order Service

1. ShopNest.OrderService.Tests xUnit project.
2. Tests: valid order, empty cart, insufficient stock, discount cap.
3. Mock IProductRepository returns stock levels.
4. Test FluentValidation CreateOrderValidator rules.
5. dotnet test --collect:"XPlat Code Coverage".

[Fact]
public async Task CreateOrderAsync_ValidCart_ReturnsOrderId()
{
    var mockProducts = new Mock<IProductRepository>();
    mockProducts.Setup(p => p.GetStockAsync(1, default)).ReturnsAsync(10);
    var mockOrders = new Mock<IOrderRepository>();
    mockOrders.Setup(o => o.AddAsync(It.IsAny<Order>(), default))
        .Callback<Order, CancellationToken>((order, _) => order.Id = Guid.NewGuid());

    var svc = new OrderService(mockOrders.Object, mockProducts.Object);
    var id = await svc.CreateOrderAsync(validDto, "cust-1");
    Assert.NotEqual(Guid.Empty, id);
}

Common errors & best practices

• Testing implementation details (private methods) — test public behavior.
• Over-mocking — mock boundaries (repos), not value objects.
• Flaky tests depending on DateTime.Now — inject IClock or freeze time.
• No Assert on exception type — use ThrowsAsync with specific type.

Interview questions

Q: [Fact] vs [Theory]?
A: Fact single case; Theory runs multiple InlineData rows.

Q: Mock vs Stub?
A: Mock verifies interactions; stub only returns canned data.

Q: What to unit test?
A: Business logic in services — not EF or HTTP pipeline (integration tests).

Q: AAA pattern?
A: Arrange setup, Act execute, Assert verify outcome.

Summary

• Unit tests protect ShopNest order business rules
• Moq isolates services from database and HTTP
• Theory tests parameterized edge cases efficiently
• Coverage guides gaps — 80% on core domain is a common target

Previous: gRPC with ASP.NET Core
Next: Integration Testing

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