Attribute Routing不起作用。

我正在使用Visual Studio Express 2013创建一个REST API。

我有两个控制器：一个用于订单，一个用于客户。

我已经创建了以下内容：

/api/clients - 获取所有客户的信息

/api/clients/1 - 获取id为1的客户的信息

/api/orders/10 - 获取id为10的订单的信息

现在，我想创建以下内容：

/api/clients/1/orders - 获取id为1的客户的所有订单信息

我已经阅读过关于属性路由的内容，但我无法使其工作。

OrdersController.cs

[EnableCors(origins: "*", headers: "*", methods: "*")]
public class OrdersController : ApiController
{
    public Order Get(string id)
    {
        // ...
    }
    [Route("clients/{id}/orders")]
    public List GetByClient(string id)
    {
        // ...
    }
}

WebApiConfig.cs

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        // 启用CORS
        config.EnableCors();
        //config.MapHttpAttributeRoutes();
        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );
    }
}

RouteConfig.cs

public class RouteConfig
{
    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
        routes.MapRoute(
            name: "Default",
            url: "{controller}/{action}/{id}",
            defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
        );
    }
}

像这样，所有其他请求都能正常工作，但是当我尝试访问/api/clients/1/orders时，我收到错误HTTP Error 404.0 - Not Found. The resource you are looking for has been removed, had its name changed, or is temporarily unavailable.

但是，只要我取消注释config.MapHttpAttributeRoutes();，我就无法访问任何请求 - 它们都返回以下结果：

    An error has occurred.

    The object has not yet been initialized. Ensure that HttpConfiguration.EnsureInitialized() is called in the application's startup code after all other initialization code.

System.InvalidOperationException

    at System.Web.Http.Routing.RouteCollectionRoute.get_SubRoutes() at System.Web.Http.Routing.RouteCollectionRoute.GetRouteData(String virtualPathRoot, HttpRequestMessage request) at System.Web.Http.WebHost.Routing.HttpWebRoute.GetRouteData(HttpContextBase httpContext)

我在这里做错了什么？