ASP.NET MVC Routing

Default route table

The application is already configured to use ASP.NET Routing when building a new ASP.NET MVC application. ASP.NET Routing is setup in two places:

1. ASP.NET Routing is enabled in the Web configuration file of your application (Web.config file). The configuration file has four sections that are crucial to routing:

• system.web.httpModules section
• system.web.httpHandlers section
• system.webserver.modules section
• system.webserver.handlers section
   

2. In the Global.asax file of the program, a route table is generated. The Global.asax file is a special file that contains ASP.NET application lifecycle event handlers. During the Application Start event, the route table is constructed.

Default Global.asax file for an ASP.NET MVC application:

//C# code
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Routing;

namespace MvcApplication1
{
    public class MvcApplication : System.Web.HttpApplication
    {
        public static void RegisterRoutes(RouteCollection routes)
        {
            routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

            routes.MapRoute(
                "Default",                                              // Route name
                "{controller}/{action}/{id}",                           // URL with parameters
                new { controller = "Home", action = "Index", id = "" }  // Parameter defaults
            );
        }
        protected void Application_Start()
        {
            RegisterRoutes(RouteTable.Routes);
        }
    }
}

The Application_Start() method is called when an MVC application first starts. This function calls the RegisterRoutes() method, which in turn calls the RegisterRoutes() method. The route table is created using the RegisterRoutes() function.
There is only one route in the default route table (named Default). The first section of a URL is mapped to a controller name, the second segment to a controller action, and the third segment to a parameter named id by the Default route.
Assume you type the following URL into the address bar of your web browser:

/Home/Index/100

Source

The Default route maps this URL to the following parameters:
controller = Home
action = Index
id = 100

When you request the URL /Home/Index/100, the following code is executed:
HomeController.Index(100)
For all three options, the Default route includes defaults. If you don't specify a controller, the controller option will be set to Home by default. The action argument defaults to the value Index if you don't specify an action. Finally, the id parameter defaults to an empty string if you don't provide one.

How the default route maps URLs to controller actions

Let's look at how the Default route binds URLs to controller actions in a few samples. 

Default Parameter

Assume you type the following URL into the address bar of your browser:
/Home
The Index() function of the HomeController class in the code below will be invoked if you enter this URL because the Default route parameter defaults.

HomeController.cs

//C# code
using System.Web.Mvc;

namespace MvcApplication1.Controllers
{
    [HandleError]
    public class HomeController : Controller
    {
        public ActionResult Index(string id)
        {
            return View();
        }
    }
}

Index() is a method in the HomeController class that receives a single parameter named Id. The Index() method is called with an empty string as the Id parameter when the URL /Home is used.

Index action with no parameter

The URL /Home also matches the Index() function of the HomeController class in the code below due to the way the MVC framework initiates controller actions.

HomeController.cs 

//C# code
using System.Web.Mvc;

namespace MvcApplication1.Controllers
{
    [HandleError]
    public class HomeController : Controller
    {
        public ActionResult Index()
        {
            return View();
        }
    }
}

There are no parameters for the Index() function in the code above. This Index() method will be invoked when the URL /Home is entered. This procedure is likewise invoked by the URL /Home/Index/100 (the Id is ignored).

Index action with nullable parameter

The Index() method of the HomeController class in the code below also matches the URL /Home.

HomeController.cs 

//C# code
using System.Web.Mvc;

namespace MvcApplication1.Controllers
{
    [HandleError]
    public class HomeController : Controller
    {
        public ActionResult Index(int? id)
        {
            return View();
        }
    }
}

The Index() function has one Integer parameter in the example above. Index() can be called without triggering an error since the parameter is nullable (it can have the value Null).

Index action with Id parameter

Finally, because the Id parameter is not a nullable parameter, using the Index() method in the code below with the URL /Home throws an exception.

//C# code
using System.Web.Mvc;

namespace MvcApplication1.Controllers
{
    [HandleError]
    public class HomeController : Controller
    {
        public ActionResult Index(int id)
        {
            return View();
        }
    }
}

In the given code, the URL /Home/Index/100, on the other hand, works perfectly with the Index controller action. The Index() method is called with an Id argument of 100 when the request /Home/Index/100 is made.

Example for ASP.NET MVC Routing 

When the application first starts up, ASP.NET MVC looks through the available assemblies for a class that implements the System.Web.Mvc. IController interface or is inherited from a class that implements this interface and whose class names end in the suffix Controller. The routing framework removes the Controller suffix from all controller class names when it utilizes this list to determine which controllers it has access to.
Assume we're working with a book entity to implement Create, Read, Update, and Delete operations. As a result, we'll utilize the table below to explain URL routing:

Table: Request's URLs that match the default route pattern

Because it satisfies every segment of the route pattern, the last request's URL (http://localhost:4736/Book/Edit/100) in the table is a perfect match to the registered default URL pattern. However, if we don't provide a complete request's URL, the routing engine automatically calls the controller and action method as per the default route pattern.

Frequently Asked Questions

What is MVC in ASP.NET?

MVC is a design pattern that separates the user interface (view), data (model), and application logic (controller). This pattern aids in the separation of concerns.

What does the Route contain?

Route contains URL pattern and handler information. URL pattern starts after the domain name.

In which class is Route configured?

The RouteConfig class can be used to configure routes. It's also possible to set up multiple custom routes.

Why routing is required in MVC?

Routing in MVC (Model-View-Controller) is essential for directing incoming requests to the appropriate controller actions based on URL patterns. It helps define the application's URL structure, enabling users to access different functionalities and ensuring a clear separation between data handling and presentation.

What happens when you don’t provide the URL’s controller, action, and id?

If you don't supply a controller, the controller parameter defaults to the value Home. Suppose you don't give an action, the action parameter defaults to the value Index. If you don't provide an id, the id parameter defaults to an empty string.

Conclusion

This article has discussed the importance of MVC routing in ASP.NET, explaining its role in directing requests to the correct controller actions and enabling cleaner, more maintainable code for user-friendly applications.But this is not enough; you need something extra to excel in web development truly. If you want to learn more about web development, you can read our articles or take our highly curated Web Development course.

Recommended Reading: 

• Features Of ASP Net
• First app in ASP.NET
• ASP.NET MVC Validations
• Spring MVC Flow
• Introduction to C#
• What is .Net