📌 Series Context: This is the seventh 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. Now, we'll explore Web API & Microservices.

Why Web API & Microservices?

Modern applications rely on APIs to expose functionality and integrate with other systems. ASP.NET Core Web API is widely used to build RESTful services. Microservices architecture takes this further by breaking applications into independent, loosely coupled services.

Interviewers often ask about REST principles, API versioning, authentication, and microservices design patterns.

REST Principles

Stateless

Resource-Based

HTTP Methods

Uniform Interface

Example: Patient Web API

Controller

[ApiController]
[Route("api/[controller]")]
public class PatientController : ControllerBase
{
    private readonly IPatientService _service;

    public PatientController(IPatientService service)
    {
        _service = service;
    }

    [HttpGet]
    public IActionResult GetAll()
    {
        return Ok(_service.GetAll());
    }

    [HttpPost]
    public IActionResult Create(Patient patient)
    {
        _service.Add(patient);
        return CreatedAtAction(nameof(GetAll), patient);
    }
}

👉 Interview Tip: Be ready to explain the difference between Controller and ControllerBase.

API Versioning

Versioning ensures backward compatibility.

services.AddApiVersioning(options =>
{
    options.DefaultApiVersion = new ApiVersion(1, 0);
    options.AssumeDefaultVersionWhenUnspecified = true;
    options.ReportApiVersions = true;
});

👉 Common Versioning Strategies:

Authentication & Security

JWT (JSON Web Tokens)

OAuth2

API Keys

Example JWT Authentication Configuration

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options =>
    {
        options.TokenValidationParameters =
            new TokenValidationParameters
            {
                ValidateIssuer = true,
                ValidateAudience = true,
                ValidateLifetime = true,
                ValidateIssuerSigningKey = true
            };
    });

Microservices Architecture

Microservices break applications into independent services that communicate through APIs or messaging systems.

Key Principles

Common Tools

Common Interview Questions (Part 7)

Conclusion

In this seventh part, we covered:

This foundation prepares you for Part 8, where we'll explore Azure & Cloud Integration—covering hosting, serverless functions, and cloud databases.

Summary

ASP.NET Core Web APIs enable applications to expose functionality through RESTful services, while microservices architecture helps organizations build scalable and independently deployable systems. Understanding REST principles, API versioning, JWT authentication, OAuth2, and microservices design patterns is essential for modern .NET development and frequently appears in technical interviews.