ASP.NET Core Basics: Web API, MVC, Routing & Middleware in C#

1. ASP.NET Core Basics

Q: What is ASP.NET Core?

ASP.NET Core is a cross-platform, high-performance, open-source framework for building modern web applications and APIs using C#. It supports building web APIs, MVC (Model-View-Controller) applications, and other web-based solutions, with features like dependency injection, middleware, and routing.

Q: Why is ASP.NET Core important?

Q: How does ASP.NET Core differ from C/C++ for web development?

2. Building Web APIs and MVC Apps

Q: What is a Web API in ASP.NET Core?

A Web API is a RESTful service that exposes endpoints to handle HTTP requests (e.g., GET, POST) and return data (e.g., JSON, XML). In ASP.NET Core, Web APIs are built using controllers or minimal APIs, leveraging attributes like [HttpGet] and [HttpPost].

Q: What is an MVC app in ASP.NET Core?

An MVC (Model-View-Controller) app follows the MVC pattern:

Q: How do you build a Web API and MVC app in ASP.NET Core?

Q: Can you give an example of a Web API and MVC app in ASP.NET Core?

Web API Example (Minimal API):

using Microsoft.AspNetCore.Builder;
using Microsoft.Extensions.DependencyInjection;

var builder = WebApplication.CreateBuilder(args);

// Register services
builder.Services.AddSingleton<IUserService, UserService>();

var app = builder.Build();

// Web API endpoints (Minimal API)
app.MapGet("/api/users/{id}", (int id, IUserService userService) =>
{
    var user = userService.GetUser(id);
    return user != null ? Results.Ok(user) : Results.NotFound();
});

app.MapPost("/api/users", (User user, IUserService userService) =>
{
    userService.AddUser(user);
    return Results.Created($"/api/users/{user.Id}", user);
});

app.Run();

// Model
public class User
{
    public int Id { get; set; }
    public string Name { get; set; } = string.Empty;
}

// Service
public interface IUserService
{
    User? GetUser(int id);
    void AddUser(User user);
}

public class UserService : IUserService
{
    private readonly List<User> _users = new();

    public User? GetUser(int id) => _users.FirstOrDefault(u => u.Id == id);

    public void AddUser(User user)
    {
        user.Id = _users.Count + 1;
        _users.Add(user);
    }
}

Description: A minimal Web API with GET and POST endpoints to retrieve and add users. Uses dependency injection for the UserService.

Usage:

MVC App Example:

using Microsoft.AspNetCore.Mvc;

var builder = WebApplication.CreateBuilder(args);

// Add MVC services
builder.Services.AddControllersWithViews();
builder.Services.AddSingleton<IUserService, UserService>();

var app = builder.Build();

// Configure middleware
app.UseRouting();
app.MapControllerRoute(
    name: "default",
    pattern: "{controller=Home}/{action=Index}/{id?}");

app.Run();

// Model
public class User
{
    public int Id { get; set; }
    public string Name { get; set; } = string.Empty;
}

// Service
public interface IUserService
{
    List<User> GetUsers();
    void AddUser(User user);
}

public class UserService : IUserService
{
    private readonly List<User> _users = new();

    public List<User> GetUsers() => _users;

    public void AddUser(User user)
    {
        user.Id = _users.Count + 1;
        _users.Add(user);
    }
}

// Controller
public class HomeController : Controller
{
    private readonly IUserService _userService;

    public HomeController(IUserService userService)
    {
        _userService = userService;
    }

    public IActionResult Index()
    {
        var users = _userService.GetUsers();
        return View(users);
    }

    [HttpPost]
    public IActionResult AddUser(User user)
    {
        if (ModelState.IsValid)
        {
            _userService.AddUser(user);
            return RedirectToAction("Index");
        }
        return View("Index", _userService.GetUsers());
    }
}

@model List<User>
<!DOCTYPE html>
<html>
<head>
    <title>User List</title>
</head>
<body>
    <h2>User List</h2>
    <form asp-action="AddUser" method="post">
        <input name="Name" placeholder="Enter name" />
        <button type="submit">Add User</button>
    </form>
    <ul>
        @foreach (var user in Model)
        {
            <li>@user.Name (ID: @user.Id)</li>
        }
    </ul>
</body>
</html>

Description: An MVC app with a home page displaying a user list and a form to add users. Uses Razor for views and dependency injection for the UserService.

Usage:

3. Routing, Middleware, and Controllers

Q: What is routing in ASP.NET Core?

Routing maps incoming HTTP requests to specific endpoints (e.g., controller actions or minimal API handlers) based on URL patterns. ASP.NET Core supports:

Q: What is middleware in ASP.NET Core?

Middleware is a chain of components that process HTTP requests and responses. Each component can handle the request, modify it, or pass it to the next component. Common middleware includes:

Q: What are controllers in ASP.NET Core?

Controllers are classes (inheriting from Controller or ControllerBase) that handle HTTP requests in MVC or Web API applications. They contain action methods (e.g., Index, Get) marked with HTTP attributes (e.g., [HttpGet]) to process requests and return responses (e.g., views, JSON).

Q: Can you give an example of routing, middleware, and controllers in ASP.NET Core?

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.DependencyInjection;

var builder = WebApplication.CreateBuilder(args);

// Register services
builder.Services.AddControllers();
builder.Services.AddSingleton<IUserService, UserService>();

var app = builder.Build();

// Middleware pipeline
app.UseRouting();
app.Use(async (context, next) =>
{
    Console.WriteLine($"Request: {context.Request.Path}");
    await next.Invoke();
});
app.UseEndpoints(endpoints => endpoints.MapControllers());

app.Run();

// Model
public class User
{
    public int Id { get; set; }
    public string Name { get; set; } = string.Empty;
}

// Service
public interface IUserService
{
    User? GetUser(int id);
    void AddUser(User user);
}

public class UserService : IUserService
{
    private readonly List<User> _users = new();

    public User? GetUser(int id) => _users.FirstOrDefault(u => u.Id == id);

    public void AddUser(User user)
    {
        user.Id = _users.Count + 1;
        _users.Add(user);
    }
}

// Controller
[ApiController]
[Route("api/[controller]")]
public class UsersController : ControllerBase
{
    private readonly IUserService _userService;

    public UsersController(IUserService userService)
    {
        _userService = userService;
    }

    [HttpGet("{id}")]
    public IActionResult GetUser(int id)
    {
        var user = _userService.GetUser(id);
        return user != null ? Ok(user) : NotFound();
    }

    [HttpPost]
    public IActionResult AddUser([FromBody] User user)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }
        _userService.AddUser(user);
        return CreatedAtAction(nameof(GetUser), new { id = user.Id }, user);
    }
}

Description: A Web API with:

Usage:

Output (console):

Request: /api/users/1

Q: How do routing, middleware, and controllers in ASP.NET Core differ from C/C++?

4. Common Mistakes & Best Practices

Q: Common mistakes?

Routing:

Middleware:

Controllers:

Q: Best practices?

Routing:

Middleware:

Controllers:

General: