.NET  

Mastering .NET Interviews – Part 9: Testing & Performance

📌 Series Context: This is the ninth article in the 10‑part series covering the most important areas of .NET interview preparation. In Part 1, we introduced the .NET ecosystem. In Part 2, we covered C# fundamentals. In Part 3, we explored advanced C# features. In Part 4, we discussed ASP.NET MVC. In Part 5, we focused on ASP.NET Core. In Part 6, we covered Entity Framework & Data Access. In Part 7, we explored Web API & Microservices. In Part 8, we discussed Azure & Cloud Integration. Now, we’ll focus on Testing & Performance.

Why Testing & Performance Matter

In interviews, employers want to know if you can deliver reliable, maintainable, and efficient code. Testing ensures correctness, while performance tuning ensures scalability. Together, they make your applications production-ready.

Unit Testing

Unit testing validates individual components.

Example with xUnit

public class CalculatorTests
{
    [Fact]
    public void Add_ReturnsCorrectSum()
    {
        var calc = new Calculator();
        var result = calc.Add(2, 3);
        Assert.Equal(5, result);
    }
}

👉 Interview Tip: Be ready to explain the difference between xUnit, NUnit, and MSTest.

Mocking & Dependency Injection

Mocking isolates dependencies during tests.

var mockService = new Mock<IPatientService>();
mockService.Setup(s => s.GetAll()).Returns(new List<Patient> { new Patient { Name = "John" } });

var controller = new PatientController(mockService.Object);
var result = controller.Index();

👉 Interview Tip: Be ready to explain why mocking is important.

Integration Testing

Integration tests validate multiple components working together.

[Fact]
public async Task GetPatients_ReturnsOk()
{
    var client = _factory.CreateClient();
    var response = await client.GetAsync("/api/patient");
    response.EnsureSuccessStatusCode();
}

Performance Tuning

Performance is critical in enterprise applications.

Techniques

  • Caching → Reduce database calls.

  • Async Programming → Prevent blocking threads.

  • Connection Pooling → Optimize database connections.

  • Profiling → Identify bottlenecks with tools like dotTrace.

Example: Caching

services.AddMemoryCache();

public class PatientService
{
    private readonly IMemoryCache _cache;

    public PatientService(IMemoryCache cache)
    {
        _cache = cache;
    }

    public IEnumerable<Patient> GetAll()
    {
        if (!_cache.TryGetValue("patients", out List<Patient> patients))
        {
            patients = LoadPatientsFromDb();
            _cache.Set("patients", patients, TimeSpan.FromMinutes(10));
        }
        return patients;
    }
}

Common Interview Questions (Part 9)

  • What is the difference between unit testing and integration testing?

  • How do you use mocking frameworks in .NET?

  • What are common performance bottlenecks in .NET applications?

  • How does caching improve performance?

  • What tools do you use for profiling and monitoring?

Conclusion

In this ninth part, we covered:

  • Unit testing with xUnit.

  • Mocking and dependency injection for isolated tests.

  • Integration testing for end-to-end validation.

  • Performance tuning techniques like caching, async programming, and profiling.

This foundation prepares you for Part 10, where we’ll explore System Design & Real-World Scenarios—applying everything you’ve learned to design scalable enterprise applications.

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