Advanced Testing Strategies in .NET Core

As .NET Core continues to mature, testing strategies must evolve to keep pace with growing application complexity. For larger-scale projects, relying solely on unit tests isn’t enough — you need integration tests, UI tests, and contract tests to ensure your app is robust and reliable across all layers.

1. Integration Tests: Verifying System Behavior End-to-End

Integration tests go beyond individual components to test how they work together — for example, testing your API endpoints with a real or in-memory database.

• Use WebApplicationFactory in ASP.NET Core to spin up your app in-memory.
• Leverage TestServer to simulate HTTP requests without network overhead.
• Use an in-memory or containerized database (like SQLite or Dockerized SQL Server) to test real DB interactions without polluting your dev or production environment.

Example:

[Fact]
public async Task GetProducts_ReturnsProductsList()
{
    var factory = new WebApplicationFactory<Startup>();
    var client = factory.CreateClient();

    var response = await client.GetAsync("/api/products");
    response.EnsureSuccessStatusCode();

    var products = JsonSerializer.Deserialize<List<Product>>(await response.Content.ReadAsStringAsync());
    Assert.NotEmpty(products);
}

2. UI Testing: End-to-End Browser Automation

Tools like Playwright and Selenium help you automate browser interactions to test user workflows.

• Write tests simulating real user behavior: clicking buttons, filling forms, navigation.
• Integrate with your CI/CD pipelines for continuous feedback.
• Combine with mocking APIs to isolate front-end tests.

3. Contract Testing: Ensuring API Compatibility

Contract tests verify that different services communicate correctly.

• Use Pact or similar tools to define consumer-provider contracts.
• Automate contract verification during CI/CD to avoid breaking changes.
• Crucial for microservices architectures to maintain seamless integration.

4. Advanced Mocking and Test Isolation

Mocks and fakes help isolate the system under test by replacing external dependencies.

• Use Moq or NSubstitute for mocking interfaces and classes.
• Prefer constructor injection to enable easy swapping of dependencies.
• Avoid over-mocking — focus on critical external interactions.

Example:

var mockRepo = new Mock<IProductRepository>();
mockRepo.Setup(repo => repo.GetAll()).Returns(new List<Product> { new Product { Id = 1, Name = "Test Product" } });

var service = new ProductService(mockRepo.Object);
var products = service.GetAllProducts();

Assert.Single(products);

Conclusion

A well-rounded test suite combining unit, integration, UI, and contract tests is essential for the health of large .NET Core applications. By using advanced mocking and test isolation techniques, you can write maintainable tests that give you confidence while evolving your codebase.