Attribute Routing in ASP.NET MVC 5

Routing is how ASP.NET MVC matches a URI to an action. MVC 5 supports a new type of routing, called attribute routing. As the name implies, attribute routing uses attributes to define routes. Attribute routing gives you more control over the URIs in your web application.

The earlier style of routing, called convention-based routing, is still fully supported. In fact, you can combine both techniques in the same project.

This post will cover the basic features and options of Attribute Routing, in ASP.NET MVC 5.

• Why Attribute Routing?
• Enabling Attribute Routing
• Optional URI Parameters and Default Values
• Route Prefixes
• Default Route
• Route Constraints
• Custom Route Constraints
• Route Names
• Areas

Why Attribute Routing?

For example, a socially enhanced e-commerce website could have the following routes:

• {productId:int}/{productTitle}
  Mapped to ProductsController.Show(int id)
• {username}
  Mapped to ProfilesController.Show(string username)
• {username}/catalogs/{catalogId:int}/{catalogTitle}
  Mapped to CatalogsController.Show(string username, int catalogId)

(Don’t mind the specific syntax right now, we will touch on this later.)   

In previous version of ASP.NET MVC, the rules would be set in the RouteConfig.cs file, and point to the actual controller actions, as such:

When the route definitions are co-located with the actions, within the same source file rather than being declared on an external configuration class, it can make it easier to reason about the mapping between URIs and actions. The previous route definition would be set using the following, simple attribute:

Enabling Attribute Routing

To enable attribute routing, call MapMvcAttributeRoutes during configuration.

You can also combine attribute routing with convention-based routing.

Optional URI Parameters and Default Values

You can make a URI parameter optional by adding a question mark to the route parameter. You can also specify a default value by using the form parameter=value.

In this example, both /books and /books/1430210079 will route to the “View” action, the former will result with listing all books, and the latter will list the specific book. Both /books/lang and /books/lang/en will be treated the same.

Route Prefixes

Often, the routes in a controller all start with the same prefix. For example:

You can set a common prefix for an entire controller by using the [RoutePrefix] attribute:

Use a tilde (~) on the method attribute to override the route prefix if needed:

Default Route

You can also apply the [Route] attribute on the controller level, capturing the action as a parameter. That route would then be applied on all actions in the controller, unless a specific [Route] has been defined on a specific action, overriding the default set on the controller.

Route Constraints

Route constraints let you restrict how the parameters in the route template are matched. The general syntax is {parameter:constraint}. For example:

Here, the first route will only be selected if the "id" segment of the URI is an integer. Otherwise, the second route will be chosen.

The following table lists the constraints that are supported.

Notice that some of the constraints, such as "min", take arguments in parentheses.

You can apply multiple constraints to a parameter, separated by a colon, for example:

Custom Route Constraints

You can create custom route constraints by implementing the IRouteConstraint interface. For example, the following constraint restricts a parameter to set of valid values:

The following code shows how to register the constraint:

Now you can apply the constraint in your routes:

Route Names

You can specify a name for a route, in order to easily allow URI generation for it. For example, for the following route:

you could generate a link using Url.RouteUrl:

Areas

You can define that a controller belongs to an area by using the [RouteArea] attribute. When doing so, you can safely remove the AreaRegistration class for that area.

With this controller, the following link generation call will result with the string "/Admin/menu/show-options"

You can set up a custom prefix for the area that defer from the area name, by using the AreaPrefix named parameter, for example:

If you are using both Areas with route attributes, and areas with convention based routes (set by an AreaRegistration class), then you need to make sure that area registration happen after MVC attribute routes are configured, however before the default convention-based route is set. The reason is that route registration should be ordered from the most specific (attributes) through more general (area registration) to the mist generic (the default route) to avoid generic routes from “hiding” more specific routes by matching incoming requests too early in the pipeline.

Example: