Explain how routing works in ASP.NET Core MVC applications

In ASP.NET Core MVC applications, routing is a key feature that maps incoming HTTP requests to the corresponding controller actions. Here's an overview of how routing works:

1. Defining Routes

Routes are defined in the Startup.cs file, specifically in the Configure method using the app.UseEndpoints method. The routes are typically set up within the UseEndpoints middleware, which is part of the request processing pipeline.

2. Route Templates

Route templates are patterns that are matched against the URL paths of incoming requests. They usually contain placeholders for parameters. Here's an example of a route template:

app.UseEndpoints(endpoints =>

{

    endpoints.MapControllerRoute(

        name: "default",

        pattern: "{controller=Home}/{action=Index}/{id?}");

});

In this example:

• controller=Home: The default controller is Home.
• action=Index: The default action method is Index.
• id?: The id parameter is optional (denoted by ?).

3. Attribute Routing

In addition to conventional routing (defined in Startup.cs), ASP.NET Core MVC supports attribute routing, where routes are defined directly on the controller actions using attributes.

[Route("products")]

public class ProductsController : Controller

{

    [Route("")]

    [Route("index")]

    public IActionResult Index()

    {

        return View();

    }

    [Route("{id}")]

    public IActionResult Details(int id)

    {

        // code to retrieve product details

        return View();

    }

}

4. Routing Middleware

The routing middleware processes the incoming requests and matches them against the defined route templates. If a match is found, the corresponding controller and action method are invoked.

5. Route Constraints

Constraints can be added to route parameters to restrict the matching criteria. For example, you can ensure a parameter is an integer:

app.UseEndpoints(endpoints =>

{

    endpoints.MapControllerRoute(

        name: "default",

        pattern: "{controller=Home}/{action=Index}/{id:int?}");

});

6. Customizing Routing

ASP.NET Core MVC allows extensive customization of routing through route constraints, custom route handlers, and middleware.

Example in Startup.cs

Here's a more comprehensive example demonstrating the setup of conventional routing in an ASP.NET Core MVC application:

public class Startup

{

    public void ConfigureServices(IServiceCollection services)

    {

        services.AddControllersWithViews();

    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)

    {

        if (env.IsDevelopment())

        {

            app.UseDeveloperExceptionPage();

        }

        else

        {

            app.UseExceptionHandler("/Home/Error");

            app.UseHsts();

        }

        app.UseHttpsRedirection();

        app.UseStaticFiles();

        app.UseRouting();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>

        {

            endpoints.MapControllerRoute(

                name: "default",

                pattern: "{controller=Home}/{action=Index}/{id?}");

        });

    }

}

In this setup:

• UseRouting adds route matching to the middleware pipeline.
• UseEndpoints defines the endpoints for routing.
• The default route maps to the HomeController and its Index action method, with an optional id parameter.

Understanding these fundamentals of routing in ASP.NET Core MVC will help you effectively handle and direct HTTP requests in your applications.