.NET Testing Web APIs with xUnit and Moq

By Sandeep Mhaske | January 2025

Table of Contents

1. Introduction
2. Why Testing Web APIs is Important
3. Overview of xUnit
4. Overview of Moq
5. Setting Up the Testing Environment
6. Example: Testing a Web API
7. Best Practices for API Testing
8. Conclusion

Introduction

Testing is a critical part of application development, ensuring that your application functions as intended. In .NET, testing Web APIs can be streamlined using tools like xUnit and Moq. This blog provides a deep dive into these tools, with practical examples to help you write efficient and maintainable tests for your Web APIs.

The following diagram illustrates testing Web APIs can be streamlined using tools like xUnit and Moq:

Why Testing Web APIs is Important

Web APIs act as the backbone for modern applications, enabling communication between different systems. Thorough testing ensures:

• Reliable functionality across various scenarios.
• Prevention of regressions during code updates.
• Validation of business logic.
• Improved code quality and maintainability.

Overview of xUnit

xUnit is a popular unit testing framework for .NET applications. It offers:

• A lightweight and extensible testing library.
• Attributes like [Fact] and [Theory] to define test cases.
• Built-in support for dependency injection.

To get started, add the xunit and xunit.runner.visualstudio NuGet packages to your test project.

Overview of Moq

Moq is a mocking framework used to create mock objects for testing purposes. It helps in:

• Isolating dependencies in unit tests.
• Simulating the behavior of interfaces and classes.
• Verifying interactions between components.

Install the Moq NuGet package in your test project to use it.

Setting Up the Testing Environment

Before writing tests, ensure your environment is ready:

1. Create a new test project using dotnet new xunit.
2. Add references to your API project.
3. Install required NuGet packages: xunit, Moq, and Microsoft.AspNetCore.Mvc.Testing.

Example: Testing a Web API

Step 1: API Controller

[ApiController]
[Route("api/[controller]")]
public class ProductsController : ControllerBase
{
    private readonly IProductService _productService;

    public ProductsController(IProductService productService)
    {
        _productService = productService;
    }

    [HttpGet("{id}")]
    public IActionResult GetProductById(int id)
    {
        var product = _productService.GetProductById(id);
        if (product == null) return NotFound();
        return Ok(product);
    }
}
            

Step 2: Write the Test

public class ProductsControllerTests
{
    private readonly Mock<IProductService> _mockProductService;
    private readonly ProductsController _controller;

    public ProductsControllerTests()
    {
        _mockProductService = new Mock<IProductService>();
        _controller = new ProductsController(_mockProductService.Object);
    }

    [Fact]
    public void GetProductById_ProductExists_ReturnsOk()
    {
        // Arrange
        var productId = 1;
        var product = new Product { Id = productId, Name = "Test Product" };
        _mockProductService.Setup(s => s.GetProductById(productId)).Returns(product);

        // Act
        var result = _controller.GetProductById(productId);

        // Assert
        var okResult = Assert.IsType<OkObjectResult>(result);
        var returnedProduct = Assert.IsType<Product>(okResult.Value);
        Assert.Equal(productId, returnedProduct.Id);
    }

    [Fact]
    public void GetProductById_ProductNotFound_ReturnsNotFound()
    {
        // Arrange
        var productId = 2;
        _mockProductService.Setup(s => s.GetProductById(productId)).Returns((Product)null);

        // Act
        var result = _controller.GetProductById(productId);

        // Assert
        Assert.IsType<NotFoundResult>(result);
    }
}
            

Best Practices for API Testing

• Mock external dependencies to isolate the unit under test.
• Test both positive and negative scenarios.
• Leverage test data builders for complex objects.
• Run tests as part of CI/CD pipelines.

Conclusion

Testing Web APIs with xUnit and Moq is a powerful approach to ensure application reliability and maintainability. By writing comprehensive test cases, you can confidently deploy updates without fear of regressions. Implement the steps and best practices discussed in this blog to enhance the quality of your .NET applications.