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The structure of the code you just saw is optimized for testability; nothing prevents you from stuffing all the code in the body of Application_Start.

The MapPageRoute method is a helper method that creates a Route object and adds it to the Routes collection. Here’s a glimpse of its internal implementation:

public Route MapPageRoute(String routeName,

String routeUrl,

String physicalFile,

Boolean checkPhysicalUrlAccess,

RouteValueDictionary defaults,

RouteValueDictionary constraints,

RouteValueDictionary dataTokens)

{

if (routeUrl == null)

{

throw new ArgumentNullException("routeUrl");

}

// Create the new route

var route = new Route(routeUrl,

defaults, constraints, dataTokens,

new PageRouteHandler(physicalFile, checkPhysicalUrlAccess));

// Add the new route to the global collection

this.Add(routeName, route);

return route;

}

The MapPageRoute method offers a simplified interface for creating a Route object. In particular, it requires you to specify the name of the route, the URL pattern for the route, and the physical ASP.NET Web Forms page the URL will map to. In addition, you can specify a Boolean flag to enforce the application of current authorization rules for the actual page. For example, imagine that the user requests a URL such as customers/edit/alfki. Imagine also that such a URL is mapped to customers.aspx and that this page is restricted to the admin role only. If the aforementioned Boolean argument is false, all users are allowed to view the page behind the URL. If the Boolean value is true, only admins will be allowed.

Finally, the MapPageRoute method can accept three dictionaries: the default values for URL parameters, additional constraints on the URL parameters, plus custom data values to pass on to the route handler.

In the previous example, we aren’t using constraints and data tokens. Instead, we are specifying default values for the categoryName and action parameters. As a result, an incoming URL such as /category will be automatically resolved as if it were /category/edit/beverages.

Programmatic Access to Route Values

The MapPageRoute method just configures routes recognized by the application. Its job ends with the startup of the application. The URL routing HTTP module then kicks in for each request and attempts to match the request URL to any of the defined routes.

Routes are processed in the order in which they have been added to the Routes collection, and the search stops at the first match. For this reason, it is extremely important that you list your routes in decreasing order of importance—stricter rules must go first.

Beyond the order of appearance, other factors affect the process of matching URLs to routes. One is the set of default values that you might have provided for a route. Default values are simply values that are automatically assigned to defined placeholders in case the URL doesn’t provide specific values. Consider the following two routes:

{Orders}/{Year}/{Month}

{Orders}/{Year}

If you assign the first route’s default values for both {Year} and {Month}, the second route will never be evaluated because, thanks to the default values, the first route is always a match regardless of whether the URL specifies a year and a month.

The URL-routing HTTP module also uses constraints (which I’ll say more about in a moment) to determine whether a URL matches a given route. If a match is finally found, the routing module gets the HTTP handler from the route handler and maps it to the HTTP context of the request.

Given the previously defined route, any matching requests are mapped to the categories.aspx page. How can this page know about the route parameters? How can this page know about the action requested or the category name? There’s no need for the page to parse (again) the URL. Route parameters are available through a new property on the Page class—the RouteData property.

RouteData is a property of type RouteData and features the members listed in Table 4-9.

Table 4-9. Members