*Routing* is how Web API matches a URI to an action. Web API 2 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 API. For example, you can easily create URIs that describe hierarchies of resources.
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 topic shows how to enable attribute routing and describes the various options for attribute routing. For an end-to-end tutorial that uses attribute routing, see [Create a REST API with Attribute Routing in Web API 2](create-a-rest-api-with-attribute-routing.md).
Alternatively, use NuGet Package Manager to install the necessary packages. From the **Tools** menu in Visual Studio, select **Library Package Manager**, then select **Package Manager Console**. Enter the following command in the Package Manager Console window:
`Install-Package Microsoft.AspNet.WebApi.WebHost`
<aid="why"></a>
## Why Attribute Routing?
The first release of Web API used *convention-based* routing. In that type of routing, you define one or more route templates, which are basically parameterized strings. When the framework receives a request, it matches the URI against the route template. (For more information about convention-based routing, see [Routing in ASP.NET Web API](routing-in-aspnet-web-api.md).
One advantage of convention-based routing is that templates are defined in a single place, and the routing rules are applied consistently across all controllers. Unfortunately, convention-based routing makes it hard to support certain URI patterns that are common in RESTful APIs. For example, resources often contain child resources: Customers have orders, movies have actors, books have authors, and so forth. It's natural to create URIs that reflect these relations:
`/customers/1/orders`
This type of URI is difficult to create using convention-based routing. Although it can be done, the results don't scale well if you have many controllers or resource types.
With attribute routing, it's trivial to define a route for this URI. You simply add an attribute to the controller action:
Here are some other patterns that attribute routing makes easy.
**API versioning**
In this example, "/api/v1/products" would be routed to a different controller than "/api/v2/products".
`/api/v1/products`
`/api/v2/products`
**Overloaded URI segments**
In this example, "1" is an order number, but "pending" maps to a collection.
`/orders/1`
`/orders/pending`
**Mulitple parameter types**
In this example, "1" is an order number, but "2013/06/16" specifies a date.
`/orders/1`
`/orders/2013/06/16`
<aid="enable"></a>
## Enabling Attribute Routing
To enable attribute routing, call **MapHttpAttributeRoutes** during configuration. This extension method is defined in the **System.Web.Http.HttpConfigurationExtensions** class.
Attribute routing can be combined with [convention-based](routing-in-aspnet-web-api.md) routing. To define convention-based routes, call the **MapHttpRoute** method.
If attribute routing is enabled, this code will throw an exception. If you upgrade an existing Web API project to use attribute routing, make sure to update this configuration code to the following:
The string "customers/{customerId}/orders" is the URI template for the route. Web API tries to match the request URI to the template. In this example, "customers" and "orders" are literal segments, and "{customerId}" is a variable parameter. The following URIs would match this template:
-`http://localhost/customers/1/orders`
-`http://localhost/customers/bob/orders`
-`http://localhost/customers/1234-5678/orders`
You can restrict the matching by using [constraints](#constraints), described later in this topic.
Notice that the "{customerId}" parameter in the route template matches the name of the *customerId* parameter in the method. When Web API invokes the controller action, it tries to bind the route parameters. For example, if the URI is `http://example.com/customers/1/orders`, Web API tries to bind the value "1" to the *customerId* parameter in the action.
Any controller methods that do not have a route attribute use convention-based routing. That way, you can combine both types of routing in the same project.
## HTTP Methods
Web API also selects actions based on the HTTP method of the request (GET, POST, etc). By default, Web API looks for a case-insensitive match with the start of the controller method name. For example, a controller method named `PutCustomers` matches an HTTP PUT request.
You can override this convention by decorating the mathod with any the following attributes:
- **[HttpDelete]**
- **[HttpGet]**
- **[HttpHead]**
- **[HttpOptions]**
- **[HttpPatch]**
- **[HttpPost]**
- **[HttpPut]**
The following example maps the CreateBook method to HTTP POST requests.
Route constraints let you restrict how the parameters in the route template are matched. The general syntax is "{parameter:constraint}". For example:
| int | Matches a 32-bit integer value. | {x:int} |
| length | Matches a string with the specified length or within a specified range of lengths. | {x:length(6)} {x:length(1,20)} |
| long | Matches a 64-bit integer value. | {x:long} |
| max | Matches an integer with a maximum value. | {x:max(10)} |
| maxlength | Matches a string with a maximum length. | {x:maxlength(10)} |
| min | Matches an integer with a minimum value. | {x:min(10)} |
| minlength | Matches a string with a minimum length. | {x:minlength(10)} |
| range | Matches an integer within a range of values. | {x:range(10,50)} |
| regex | Matches a regular expression. | {x:regex(^\d{3}-\d{3}-\d{4}$)} |
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.
You can create custom route constraints by implementing the **IHttpRouteConstraint** interface. For example, the following constraint restricts a parameter to a non-zero integer value.
You can also replace the entire **DefaultInlineConstraintResolver** class by implementing the **IInlineConstraintResolver** interface. Doing so will replace all of the built-in constraints, unless your implementation of **IInlineConstraintResolver** specifically adds them.
<aid="optional"></a>
## Optional URI Parameters and Default Values
You can make a URI parameter optional by adding a question mark to the route parameter. If a route parameter is optional, you must define a default value for the method parameter.
This is almost the same as the previous example, but there is a slight difference of behavior when the default value is applied.
- In the first example ("{lcid?}"), the default value of 1033 is assigned directly to the method parameter, so the parameter will have this exact value.
- In the second example ("{lcid=1033}"), the default value of "1033" goes through the model-binding process. The default model-binder will convert "1033" to the numeric value 1033. However, you could plug in a custom model binder, which might do something different.
(In most cases, unless you have custom model binders in your pipeline, the two forms will be equivalent.)
<aid="route-names"></a>
## Route Names
In Web API, every route has a name. Route names are useful for generating links, so that you can include a link in an HTTP response.
To specify the route name, set the **Name** property on the attribute. The following example shows how to set the route name, and also how to use the route name when generating a link.
When the framework tries to match a URI with a route, it evaluates the routes in a particular order. To specify the order, set the **RouteOrder** property on the route attribute. Lower values are evaluated first. The default order value is zero.
Here is how the total ordering is determined:
1. Compare the **RouteOrder** property of the route attribute.
2. Look at each URI segment in the route template. For each segment, order as follows:
1. Literal segments.
2. Route parameters with constraints.
3. Route parameters without constraints.
4. Wildcard parameter segments with constraints.
5. Wildcard parameter segments without constraints.
3. In the case of a tie, routes are ordered by a case-insensitive ordinal string comparison ([OrdinalIgnoreCase](https://msdn.microsoft.com/en-us/library/system.stringcomparer.ordinalignorecase.aspx)) of the route template.
Here is an example. Suppose you define the following controller:
Notice that "details" is a literal segment and appears before "{id}", but "pending" appears last because the **RouteOrder** property is 1. (This example assumes there are no customers named "details" or "pending". In general, try to avoid ambiguous routes. In this example, a better route template for `GetByCustomer` is "customers/{customerName}" )
