When developing Minimal APIs in .NET 8, testing is a critical step to ensure your APIs are reliable, scalable, and maintainable. Well-structured unit and integration testing can significantly improve the quality of your APIs, helping you catch bugs early and guarantee that your code behaves as expected across various scenarios.

In this blog post, we'll walk through how to implement unit and integration testing for Minimal APIs in .NET 8, covering best practices to help you deliver high-quality APIs.

Why Testing Minimal APIs is Essential

Minimal APIs, while lightweight and easy to implement, require thorough testing, just like any other API. Unit tests validate the behavior of individual components or methods, while integration tests ensure that the entire system, including external dependencies like databases or other APIs, works correctly.

Testing your Minimal APIs offers several benefits:

Preventing regressions when making updates or refactoring.
Improving maintainability by ensuring each component works as expected.
Reducing bugs by catching issues early in the development process.
Ensuring reliability across different environments and use cases.

1. Setting Up Unit Testing for Minimal APIs

Unit testing involves testing the smallest parts of your application—typically individual methods or functions—isolated from their dependencies.

Step 1: Create a Unit Test Project

To start, create a unit test project in your solution. If you're using xUnit (one of the most popular testing frameworks for .NET), you can create a test project using the following command:

dotnet new xunit -n MinimalApiTests

Add the reference to your Minimal API project in the test project:

dotnet add reference ../MinimalApiProject/MinimalApiProject.csproj

Step 2: Mock Dependencies

Since unit tests should run in isolation, you'll need to mock dependencies such as database access or external services. For mocking, you can use Moq, a popular mocking library:

dotnet add package Moq

Here’s an example of how to mock a service and test a method that depends on it:

public interface IWeatherService
{
    Task<string> GetWeatherAsync();
}

public class WeatherApi
{
    private readonly IWeatherService _weatherService;

    public WeatherApi(IWeatherService weatherService)
    {
        _weatherService = weatherService;
    }

    public async Task<string> GetWeather()
    {
        return await _weatherService.GetWeatherAsync();
    }
}

For unit testing:

public class WeatherApiTests
{
    [Fact]
    public async Task GetWeather_ReturnsExpectedWeather()
    {
        // Arrange
        var mockWeatherService = new Mock<IWeatherService>();
        mockWeatherService.Setup(service => service.GetWeatherAsync()).ReturnsAsync("Sunny");
        var weatherApi = new WeatherApi(mockWeatherService.Object);

        // Act
        var result = await weatherApi.GetWeather();

        // Assert
        Assert.Equal("Sunny", result);
    }
}

This test mocks the IWeatherService and ensures that the WeatherApi.GetWeather() method returns the expected result without actually calling the real weather service.

2. Writing Integration Tests for Minimal APIs

Unlike unit tests, integration tests ensure that different parts of the system work together, including external dependencies such as databases, third-party services, or file systems. Integration tests are often broader in scope and are crucial for testing the real behavior of your Minimal APIs.

Step 1: Use WebApplicationFactory for Integration Tests

.NET makes it easy to write integration tests for Minimal APIs using WebApplicationFactory. This factory sets up a test server to simulate the actual API environment, allowing you to run requests against your API and validate the responses.

Install the necessary testing packages:

dotnet add package Microsoft.AspNetCore.Mvc.Testing
dotnet add package Microsoft.EntityFrameworkCore.InMemory

Then, create an integration test:

public class WeatherApiIntegrationTests : IClassFixture<WebApplicationFactory<Program>>
{
    private readonly HttpClient _client;

    public WeatherApiIntegrationTests(WebApplicationFactory<Program> factory)
    {
        _client = factory.CreateClient();
    }

    [Fact]
    public async Task GetWeather_ReturnsExpectedResult()
    {
        // Act
        var response = await _client.GetAsync("/weather");
        response.EnsureSuccessStatusCode();
        var result = await response.Content.ReadAsStringAsync();

        // Assert
        Assert.Equal("Sunny", result);
    }
}

In this example, WebApplicationFactory spins up a test instance of your API, and you can send HTTP requests to it using HttpClient.

Step 2: Mock External Dependencies in Integration Tests

For integration tests, it’s important to mock external dependencies like databases to ensure that your tests run consistently without requiring a real database connection. You can use InMemory databases or mock services.

Example of configuring an InMemory database:

public class StartupTest
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddDbContext<AppDbContext>(options =>
            options.UseInMemoryDatabase("TestDb"));
    }
}

This allows you to test your API's interaction with the database in an isolated environment.

3. Best Practices for Unit and Integration Testing

To ensure your tests are effective and maintainable, follow these best practices:

1. Write Tests for Both Happy and Unhappy Paths

Test not only the expected scenarios (happy paths) but also edge cases, error handling, and invalid inputs (unhappy paths).

2. Keep Unit Tests Isolated

Ensure unit tests are isolated from external systems by using mock objects and dependency injection.

3. Automate Your Tests in CI/CD

Run your tests automatically on every commit using Continuous Integration (CI) tools like GitHub Actions, Azure Pipelines, or Jenkins.

4. Ensure Fast Feedback

Keep unit tests lightweight and fast. Integration tests may take longer, but ensure they run efficiently by focusing on core interactions.

5. Focus on Readability and Maintainability

Write clear, descriptive test names and keep your tests small and focused. This improves readability and helps future developers understand the purpose of each test.

6. Use In-Memory Databases for Integration Tests

To avoid complex setups for databases in integration tests, use In-Memory databases like SQLite or Entity Framework InMemory for simulating real database behavior.

7. Prioritize Test Coverage

Aim for high test coverage, but also ensure that your tests are meaningful. Focus on critical business logic and API endpoints.

4. Running Tests and Analyzing Results

Running tests in .NET is straightforward with the dotnet test command:

dotnet test

This command runs all your unit and integration tests and displays the results in the console. For detailed reports, you can integrate with test reporting tools or CI/CD platforms that provide a graphical view of your test results.

Conclusion

Testing is a vital aspect of building high-quality APIs. By following the strategies outlined in this guide for unit and integration testing, you can ensure that your Minimal APIs in .NET 8 are robust, maintainable, and ready for production. From mocking dependencies in unit tests to using WebApplicationFactory for integration tests, each layer of testing plays a crucial role in the stability of your APIs.

Make sure you adopt these best practices for your projects, and you'll see improvements in code quality and confidence in your API deployments.

By mastering unit and integration testing for Minimal APIs, you’re on your way to building reliable, high-performance systems that can scale efficiently. Happy coding!

Mastering Unit and Integration Testing for Minimal APIs in .NET 8: Best Practices for High-Quality Code