ASP.NET Routing Fundamentals

Understanding ASP.NET Routing

ASP.NET Routing is a powerful feature that allows you to define custom URLs for your web applications. Instead of relying on physical file paths, routing enables you to map incoming request URLs to specific handlers (like controller actions in MVC or signal handlers in Web Forms).

What is Routing?

At its core, routing is a mechanism for matching a URL request to a route definition. A route definition consists of a URL pattern and a handler that will process the request when the pattern matches. This provides greater flexibility in designing your application's URL structure, making it more user-friendly, search engine-friendly, and maintainable.

Key Concepts

Route Table: A collection of all route definitions in your application.
Route Definition: Comprises a URL pattern, default values, and constraints.
URL Pattern: A template that defines the structure of the URL to be matched. It can include parameters.
Route Parameters: Placeholders in the URL pattern (e.g., {controller}, {action}, {id}) that capture parts of the URL.
Constraints: Rules applied to route parameters to ensure they match specific criteria (e.g., an ID must be a number).
Default Values: Values that are used if a parameter is not present in the URL.
Handler: The code that executes when a route is matched (e.g., a controller action).

How Routing Works

When a request arrives at your ASP.NET application, the routing engine iterates through the registered routes in the route table. It attempts to match the incoming URL against the URL pattern of each route. The first route that successfully matches the URL is selected.

Once a route is matched, the parameters from the URL are extracted. These parameters are then used to determine which handler should execute. For example, in an MVC application, parameters like {controller} and {action} are used to select the appropriate controller and action method.

Defining Routes

Routes are typically defined in the App_Start folder within a file like RouteConfig.cs. Here's a common example of defining a default route:


using System.Web.Mvc;
using System.Web.Routing;

public class RouteConfig
{
    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

        routes.MapRoute(
            name: "Default",
            url: "{controller}/{action}/{id}",
            defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
        );
    }
}

URL Pattern Breakdown

{controller}: Matches any string that will be used as the controller name.
{action}: Matches any string that will be used as the action method name.
{id}: Matches any string that will be used as an identifier, and it's optional.

The defaults object provides fallback values. If no controller is specified in the URL, it defaults to "Home". If no action is specified, it defaults to "Index". The id parameter is marked as UrlParameter.Optional, meaning it can be omitted entirely.

Benefits of Using Routing

SEO Friendly URLs: Create clean, descriptive URLs that search engines can easily understand and index.
Maintainability: Decouple your URL structure from your physical file structure, allowing you to refactor your code without breaking existing URLs.
User Experience: Provide more intuitive and readable URLs for your users.
API Design: Essential for designing clean and RESTful APIs.

MSDN Documentation