Routing in ASP.NET Core MVC

1. Conventional Routing: One Pattern Rules All

Set in Program.cs:

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

{id?} = optional. =Home = default if missing.

2. Attribute Routing: Custom URLs

Put route directly on action:

public class BlogController : Controller
{
    [Route("blog/{year:int}/{month:int}/{slug}")]
    public IActionResult Post(int year, int month, string slug)
    {
        // Matches: /blog/2026/1/aspnet-core-routing
        return View();
    }
}

Why? SEO-friendly URLs. /blog/2026/1/my-post beats /blog/post?year=2026&month=1&slug=my-post

3. Route Constraints: Filter Bad URLs

{id:int} = only match if id is number. /product/details/abc → 404, not crash.

1. Routing Pipeline: How URL Becomes Action

Request GET /product/details/5:

1. EndpointRoutingMiddleware: Parses URL, checks route table. Sets HttpContext.GetRouteData()
2. Route Values: {controller="Product", action="Details", id="5"}
3. EndpointMiddleware: Finds ProductController.Details(int id) method
4. ActionInvoker: Binds "5" → int id = 5, calls method

What is Endpoint? Final destination. Routing picks it, Middleware executes it.

2. Order Matters in Conventional Routes

Routes are checked top-down. First match wins.

app.MapControllerRoute(
    name: "blog",
    pattern: "blog/{year}/{month}/{slug}",
    defaults: new { controller = "Blog", action = "Post" });

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

Wrong order: Put "default" first → /blog/2026/1/test matches controller=blog, action=2026. 404. Specific routes first, catch-all last.

3. Attribute Routing + Conventional = Can Mix

You need app.MapControllers(); in Program.cs to enable attributes.

// Program.cs
app.MapControllerRoute(name: "default", pattern: "{controller=Home}/{action=Index}/{id?}");
app.MapControllers(); // Scans for [Route] attributes

// Controller
[Route("api/[controller]")] // Token replacement: [controller] = Product
public class ProductController : ControllerBase
{
    [HttpGet("{id:int}")] // Combines: /api/product/5
    public IActionResult Get(int id) { }
}

4. URL Generation: Link Generation & Redirects

Never hardcode <a href="/Product/Details/5">. Route changes break it.

Why: If you change route to [Route("items/{id}")], all links auto-update.

5. Areas: Split Large Apps

/Admin/Product/Index vs /Product/Index. Same controller name, different area.

// Folder: Areas/Admin/Controllers/ProductController.cs
[Area("Admin")]
public class ProductController : Controller
{
    public IActionResult Index() { } // URL: /Admin/Product/Index
}

// Program.cs
app.MapControllerRoute(
    name: "areas",
    pattern: "{area:exists}/{controller=Home}/{action=Index}/{id?}");