Controllers & Actions in ASP.NET Core

1. Anatomy of an Action

public class ProductController : Controller
{
    // GET: /Product/Details/5
    public IActionResult Details(int id)
    {
        var product = _db.Products.Find(id); // Get data
        if (product == null) return NotFound(); // 404
        return View(product); // 200 + HTML
    }
}

3 Parts: 1. Route matches → 2. Method runs → 3. IActionResult returned.

2. Common IActionResult Types

3. Parameters: How Data Gets Into Actions

URL: /product/details/5?sort=name

public IActionResult Details(int id, string sort)
{
    // id = 5 from route: {controller}/{action}/{id}
    // sort = "name" from query string?sort=name
}

Rule: Route values → Query string → Form data. ASP.NET auto-binds if names match.

4. GET vs POST: The Golden Rule

[HttpGet] // Show the form
public IActionResult Create()
{
    return View();
}

[HttpPost] // Handle form submit
public IActionResult Create(Product product)
{
    if (!ModelState.IsValid) return View(product); // Show errors
    _db.Products.Add(product);
    _db.SaveChanges();
    return RedirectToAction("Index"); // PRG Pattern
}

1. Action Selection: How Routing Picks Your Method

Request POST /product/edit/5. ASP.NET checks:

1. Route Template: {controller=Product}/{action=Edit}/{id?} → Matches
2. HTTP Verb: [HttpPost] attribute matches POST
3. Parameters: Can id bind to int? Yes → Select action

If 2 actions match, you get AmbiguousMatchException. Fix: Use [HttpGet] / [HttpPost].

2. Model Binding: Deep Dive

ASP.NET creates action parameters from 4 sources, in order:

// POST /api/products with JSON body
[HttpPost]
public IActionResult Create([FromBody] ProductDto dto)
{
    // dto auto-filled from JSON. Needs [ApiController] for auto-validation
}

3. Async Actions: Why Task<IActionResult>

Any DB or API call must be async. Otherwise thread blocked.

// BAD: Blocks thread for 2 seconds
public IActionResult Index() {
    var products = _db.Products.ToList(); // Sync DB call
    return View(products);
}

// GOOD: Frees thread during DB wait
public async Task<IActionResult> Index() {
    var products = await _db.Products.ToListAsync(); // Async
    return View(products);
}

What is a thread? Worker that handles 1 request. Server has ~1000 threads. If 1000 users hit sync DB calls, 1001st user waits. Async = thread works on other requests during DB wait.

4. Filters: Run Code Before/After Actions

Filters = Attributes. They wrap your action like middleware. Order: Authorization → Resource → Action → Exception → Result.

5. Controller Lifecycle & DI

Controllers are Scoped by default. New instance per HTTP request.

public class ProductController : Controller
{
    private readonly AppDbContext _db;

    // Constructor injection. DI container gives you DbContext
    public ProductController(AppDbContext db) {
        _db = db; // Same instance used entire request
    }
}

Why not Singleton? DbContext isn't thread-safe. Scoped = 1 per request = safe.