Routing in ASP.NET Core MVC

1. Attribute Routing: The Only Way That Matters

Put routes directly on controllers and actions. This is required for Web APIs and best for MVC.

[Route("products")]
public class ProductsController : Controller
{
    [HttpGet] // GET /products
    public IActionResult Index() => View();

    [HttpGet("{id:int}")] // GET /products/5
    public IActionResult Details(int id) => View();

    [HttpPost] // POST /products from form
    public IActionResult Create(ProductViewModel vm) { }
}

2. Route Constraints = Input Validation for URLs

Stops /products/abc from crashing your int id action.

3. HTTP Verbs: GET, POST, PUT, DELETE

[HttpGet("products")] // List all
[HttpGet("products/{id}")] // Get one
[HttpPost("products")] // Create new
[HttpPut("products/{id}")] // Update existing
[HttpDelete("products/{id}")] // Remove

Rule: Forms use GET/POST. APIs use all 4. If you forget [HttpPost], your form gets 405 Method Not Allowed.

4. Route Order: Specific → General

First match wins. Put /blog/archive/{year} before /blog/{slug} or "archive" becomes a slug.

1. Conventional vs Attribute Routing

Rule: Web API requires attribute routing. MVC sites can use both, but attribute is cleaner.

2. Attribute Routing Deep Dive

Tokens auto-replace: [controller] becomes "Products" in ProductsController.

[Route("api/[controller]")] // api/products
public class ProductsController : ControllerBase
{
    [HttpGet("{id:int:min(1)}")] // GET api/products/5
    public IActionResult Get(int id) { }

    [HttpGet("search")] // GET api/products/search?q=phone
    public IActionResult Search([FromQuery] string q) { }
}

Multiple constraints chain with :. :int:min(1) = integer AND >= 1.

3. Route Ambiguity & Resolution

ASP.NET Core scores routes. Most specific wins. Tie = exception.

// These conflict
[HttpGet("{id}")] // Matches anything
public IActionResult GetById(string id) { }

[HttpGet("{slug}")] // Also matches anything
public IActionResult GetBySlug(string slug) { }

// Fix: Make them specific
[HttpGet("{id:int}")]
public IActionResult GetById(int id) { }

[HttpGet("{slug:alpha}")]
public IActionResult GetBySlug(string slug) { }

4. Areas: Multi-Tenant Apps

Separate MVC folders for /Admin, /Customer. Each has own Controllers/Views.

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

// Areas/Admin/Controllers/DashboardController.cs
[Area("Admin")]
[Route("admin/[controller]")]
public class DashboardController : Controller
{
    public IActionResult Index() => View(); // URL: /admin/dashboard
}

5. Catch-All & Optional Parameters

[Route("files/{*path}")] // * = catch-all
public IActionResult Download(string path)
{
    // /files/images/2024/logo.png → path="images/2024/logo.png"
}

[Route("blog/{year}/{month?}")] //? = optional
public IActionResult Posts(int year, int? month) { }
// Matches /blog/2024 and /blog/2024/06

Rule: Optional/catch-all must be last segment. {id?}/{slug} won't compile.

6. Link Generation: Tag Helpers vs Url.Action

Never hardcode URLs. If you rename [Route("products")] to [Route("items")], hardcoded links break.

<!-- Tag Helper - Preferred -->
<a asp-controller="Products" asp-action="Details" asp-route-id="@p.Id">View</a>

<!-- Url.Action -->
<a href="@Url.Action("Details", "Products", new { id = p.Id })">View</a>

<!-- Both generate: /products/5 -->