1. Why MVC? Stop Spaghetti Code

Old ASP.NET WebForms: 1.aspx file = UI + DB + logic. Designer breaks SQL. Can't test. Can't scale teams. Prerna changes button color, crashes database call.

MVC fix: 3 files, 3 jobs. Frontend team edits Views. Backend team edits Models. Easy to test each piece.

2. Request Flow: GET /products

Browser: GET /products
    ↓
Routing: Matches "{controller=Product}/{action=Index}"
    ↓
ProductController.Index(): var products = _db.Products.ToList();
    ↓
return View(products); // Looks for Views/Product/Index.cshtml
    ↓
View: @model List<Product> → foreach → <table>
    ↓
Browser: Gets final HTML. No C# visible.

3. Build It: Your First Page in 3 Files

Goal: Show "Welcome TechStore" at /home/welcome

File 1: Controllers/HomeController.cs

using Microsoft.AspNetCore.Mvc;

namespace TechStore.Controllers
{
    public class HomeController : Controller
    {
        public IActionResult Welcome()
        {
            ViewBag.Message = "Welcome TechStore";
            return View(); // Finds Views/Home/Welcome.cshtml
        }
    }
}

File 2: Views/Home/Welcome.cshtml

@{
    ViewData["Title"] = "Welcome";
}

@ViewBag.Message
Your first MVC page works!

Convention: HomeController + Welcome() = Views/Home/Welcome.cshtml. ASP.NET finds it automatically.

File 3: Model? Not needed yet

No database data = no Model. Controller → View is valid MVC. We'll cover Models in the next tutorial: Models & ViewModels.

1. MVC Is Middleware, Not Magic

Check Program.cs in.NET 6+:

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddControllersWithViews(); // Adds MVC services

var app = builder.Build();
app.UseRouting(); // Matches URL to endpoint
app.UseEndpoints(endpoints => {
    endpoints.MapControllerRoute(
        name: "default",
        pattern: "{controller=Home}/{action=Index}/{id?}");
});
app.Run();

Flow: Request → EndpointRoutingMiddleware sets RouteData → EndpointMiddleware runs ControllerActionInvoker → Your action executes.

2. Controller vs ControllerBase vs POCO

3. The 4 Deadly MVC Sins in Interviews

1. Fat Controller: 200-line actions with SQL. Fix: Move to Service layer. Controller: get data → return View. 5 lines max.
2. ViewBag Everywhere: ViewBag.User, ViewBag.Products, ViewBag.Config. No IntelliSense. Runtime crash if typo. Fix: @model ProductListViewModel. Strongly typed.
3. Async Void: public async void Index(). Request completes before DB finishes. Exception kills process. Always async Task<IActionResult>.
4. EF Entity to View: return View(db.Products.ToList()). Exposes Product.CostPrice to HTML. Security leak. Fix: Map to ProductViewModel with only public fields.

4. Model vs ViewModel vs Entity

Rule: Controller gets Entity from DB → maps to ViewModel → sends VM to View. Never Entity → View.

5. When NOT to Use MVC