MVC

MVC is a way of splitting the code that builds a web page into three parts. The letters stand for model, view and controller.

A controller is a class that handles requests. Each public method on it is an action, and one action handles one kind of request. A view is a file of HTML with holes in it, ending in .cshtml. A model, in this context, is one object holding everything that view needs to fill its holes. The view only reads the model. It never fetches data of its own.

Turning it on

builder.Services.AddControllersWithViews() registers the services MVC needs, including the engine that renders .cshtml files. app.MapControllerRoute connects URLs to actions. The usual pattern, {controller=Home}/{action=Index}/{id?}, sends /products/detail/4 to the Detail action of ProductsController with id set to 4.

Returning a view

An action returns View(model), which builds a ViewResult that the framework then renders. The view name defaults to the action name, so View(model) inside Index is shorthand for View("Index", model). They are the same call. Either way the search order is Views/{ControllerName}/{ViewName}.cshtml first, then Views/Shared/{ViewName}.cshtml. A GET of / in the example below returns status 200 and the rendered HTML of Views/Home/Index.cshtml.

What trips people up

The view is found by file name while the request runs, not when the project is built. A missing or misspelled file throws InvalidOperationException and lists every path it searched.

// Program.cs of a web project (<Project Sdk="Microsoft.NET.Sdk.Web">).
using Microsoft.AspNetCore.Mvc;

var builder = WebApplication.CreateBuilder(args);

// Controllers plus the Razor view engine.
builder.Services.AddControllersWithViews();

var app = builder.Build();

// A URL with no segments is treated as /Home/Index.
app.MapControllerRoute(
    name: "default",
    pattern: "{controller=Home}/{action=Index}/{id?}");

app.Run();

// A controller is a public class whose name ends in Controller.
public class HomeController : Controller
{
    public IActionResult Index()
    {
        // The model: one object holding everything the view needs.
        var greeting = new Greeting("Ada", 3);

        // Shorthand. The view name comes from the action name, Index.
        return View(greeting);

        // Longhand, identical: return View("Index", greeting);
    }
}

// A record is a short way to declare a class that only holds data.
public record Greeting(string Name, int Visits);

@* Views/Home/Index.cshtml *@
@model Greeting

<h1>Hello, @Model.Name</h1>
<p>Visits so far: @Model.Visits</p>

@* Rendered for the Greeting("Ada", 3) above:
<h1>Hello, Ada</h1>
<p>Visits so far: 3</p>
*@

using Microsoft.AspNetCore.Mvc;

public class ReportsController : Controller
{
    public IActionResult Daily()
    {
        // One dictionary, two spellings. ViewBag.Title is shorthand
        // for ViewData["Title"], and both read and write the same entry.
        ViewData["Title"] = "Daily report";
        ViewBag.Rows = 12;

        // An explicit name. Views/Reports/_Report.cshtml is tried first,
        // then Views/Shared/_Report.cshtml.
        return View("_Report");
    }
}

using Microsoft.AspNetCore.Mvc;

public class CartController : Controller
{
    public IActionResult Item(int id)
    {
        if (id <= 0)
        {
            return NotFound();                       // 404, no view
        }

        return RedirectToAction(nameof(Index));      // 302 to /Cart/Index
    }

    // Expression-bodied member: the same as { return View(); }
    public IActionResult Index() => View();
}