Question

I am currently working on a ASP.NET MVC 4 Web Application project that must adhere to the following design decisions:

• The main MVC application resides in the root of the solution.
• All administrator functionality resides in a separate area.
• Each external party (e.g. suppliers) has its own area.
• Each area, including the root, constitutes a well separated
  functional block. Functionality from one area may not be exposed to
  another area. This is to prevent unauthorized access of data.
• Each area, including the root, has its own RESTfull API (Web API).

All normal controllers in all areas, including the root, work as expected. However, some of my Web API controllers exhibit unexpected behaviour. For instance, having two Web API controllers with the same name but in different areas produces the following exception:

Multiple types were found that match the controller named ‘clients’.
This can happen if the route that services this request
(‘api/{controller}/{id}’) found multiple controllers defined with the
same name but differing namespaces, which is not supported.

The request for ‘clients’ has found the following matching controllers:
MvcApplication.Areas.Administration.Controllers.Api.ClientsController
MvcApplication.Controllers.Api.ClientsController

This seems strange since I have distinct routes that should separate both. Here is my AreaRegistration for the Administration section:

public class AdministrationAreaRegistration : AreaRegistration
{
    public override string AreaName
    {
        get
        {
            return "Administration";
        }
    }

    public override void RegisterArea(AreaRegistrationContext context)
    {
        context.Routes.MapHttpRoute(
            name: "Administration_DefaultApi",
            routeTemplate: "Administration/api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );

        context.MapRoute(
            "Administration_default",
            "Administration/{controller}/{action}/{id}",
            new { action = "Index", id = UrlParameter.Optional }
        );
    }
}

Moreover, I notice that I can access Area specific Web API’s while omitting the name of the area from the call.

What is going on here?
How do I get my Web API Controllers to behave just like normal ASP.NET MVC controllers?

Answer 1

ASP.NET MVC 4 does not support the partitioning of Web API controllers across Areas.

You may place WebApi controllers in different Api folders in different Areas, but ASP.NET MVC will treat as if they are all in the same place.

Fortunately, you can overcome this limitation by overriding a part of the ASP.NET MVC infrastructure. For more information about the limitation and the solution, please read my blog post ‘ASP.NET MVC 4 RC: Getting WebApi and Areas to play nicely‘. If you are only interested in the solution, read on:

Step 1. Make your routes Area aware

Add the following extension methods to your ASP.NET MVC application and make sure they are accessible from your AreaRegistration classes:

public static class AreaRegistrationContextExtensions
{
    public static Route MapHttpRoute(this AreaRegistrationContext context, string name, string routeTemplate)
    {
        return context.MapHttpRoute(name, routeTemplate, null, null);
    }

    public static Route MapHttpRoute(this AreaRegistrationContext context, string name, string routeTemplate, object defaults)
    {
        return context.MapHttpRoute(name, routeTemplate, defaults, null);
    }

    public static Route MapHttpRoute(this AreaRegistrationContext context, string name, string routeTemplate, object defaults, object constraints)
    {
        var route = context.Routes.MapHttpRoute(name, routeTemplate, defaults, constraints);
        if (route.DataTokens == null)
        {
            route.DataTokens = new RouteValueDictionary();
        }
        route.DataTokens.Add("area", context.AreaName);
        return route;
    }
}

To use the new extension method, remove the Routes property from the call chain:

context.MapHttpRoute( /* <-- .Routes removed */
    name: "Administration_DefaultApi",
    routeTemplate: "Administration/api/{controller}/{id}",
    defaults: new { id = RouteParameter.Optional }
);

Step 2. Make Web API controller selector Area aware

Add the following class to your ASP.NET MVC application and make sure it is accessible from the Global.asax

namespace MvcApplication.Infrastructure.Dispatcher
{
    using System;
    using System.Collections.Concurrent;
    using System.Collections.Generic;
    using System.Globalization;
    using System.Linq;
    using System.Net.Http;
    using System.Web.Http;
    using System.Web.Http.Controllers;
    using System.Web.Http.Dispatcher;

    public class AreaHttpControllerSelector : DefaultHttpControllerSelector
    {
        private const string AreaRouteVariableName = "area";

        private readonly HttpConfiguration _configuration;
        private readonly Lazy<ConcurrentDictionary<string, Type>> _apiControllerTypes;

        public AreaHttpControllerSelector(HttpConfiguration configuration)
            : base(configuration)
        {
            _configuration = configuration;
            _apiControllerTypes = new Lazy<ConcurrentDictionary<string, Type>>(GetControllerTypes);
        }

        public override HttpControllerDescriptor SelectController(HttpRequestMessage request)
        {
            return this.GetApiController(request);
        }

        private static string GetAreaName(HttpRequestMessage request)
        {
            var data = request.GetRouteData();
            if (data.Route.DataTokens == null)
            {
                return null;
            } 
            else 
            {
                object areaName;
                return data.Route.DataTokens.TryGetValue(AreaRouteVariableName, out areaName) ? areaName.ToString() : null;
            }
        }

        private static ConcurrentDictionary<string, Type> GetControllerTypes()
        {
            var assemblies = AppDomain.CurrentDomain.GetAssemblies();

            var types = assemblies
                .SelectMany(a => a
                    .GetTypes().Where(t =>
                        !t.IsAbstract &&
                        t.Name.EndsWith(ControllerSuffix, StringComparison.OrdinalIgnoreCase) &&
                        typeof(IHttpController).IsAssignableFrom(t)))
                .ToDictionary(t => t.FullName, t => t);

            return new ConcurrentDictionary<string, Type>(types);
        }

        private HttpControllerDescriptor GetApiController(HttpRequestMessage request)
        {
            var areaName = GetAreaName(request);
            var controllerName = GetControllerName(request);
            var type = GetControllerType(areaName, controllerName);

            return new HttpControllerDescriptor(_configuration, controllerName, type);
        }

        private Type GetControllerType(string areaName, string controllerName)
        {
            var query = _apiControllerTypes.Value.AsEnumerable();

            if (string.IsNullOrEmpty(areaName))
            {
                query = query.WithoutAreaName();
            }
            else
            {
                query = query.ByAreaName(areaName);
            }

            return query
                .ByControllerName(controllerName)
                .Select(x => x.Value)
                .Single();
        }
    }

    public static class ControllerTypeSpecifications
    {
        public static IEnumerable<KeyValuePair<string, Type>> ByAreaName(this IEnumerable<KeyValuePair<string, Type>> query, string areaName)
        {
            var areaNameToFind = string.Format(CultureInfo.InvariantCulture, ".{0}.", areaName);

            return query.Where(x => x.Key.IndexOf(areaNameToFind, StringComparison.OrdinalIgnoreCase) != -1);
        }

        public static IEnumerable<KeyValuePair<string, Type>> WithoutAreaName(this IEnumerable<KeyValuePair<string, Type>> query)
        {
            return query.Where(x => x.Key.IndexOf(".areas.", StringComparison.OrdinalIgnoreCase) == -1);
        }

        public static IEnumerable<KeyValuePair<string, Type>> ByControllerName(this IEnumerable<KeyValuePair<string, Type>> query, string controllerName)
        {
            var controllerNameToFind = string.Format(CultureInfo.InvariantCulture, ".{0}{1}", controllerName, AreaHttpControllerSelector.ControllerSuffix);

            return query.Where(x => x.Key.EndsWith(controllerNameToFind, StringComparison.OrdinalIgnoreCase));
        }
    }
}

Override the DefaultHttpControllerSelector by adding the following line to the Application_Start method in the Global.asax.

GlobalConfiguration.Configuration.Services.Replace(typeof(IHttpControllerSelector), new AreaHttpControllerSelector(GlobalConfiguration.Configuration));

Congratulations, your Web API controllers will now respect the rules of your areas just like your normal MVC controllers do!

UPDATE: 6 september 2012

Several developers have contacted me about a scenario they encountered where the DataTokens property of the route variable is null. My implementation assumes that the DataTokens property is always initialized and will not function properly if this property is null. This behavior is most likely caused by recent changes in the ASP.NET MVC framework and may be actually be a bug in the framework. I’ve updated my code to handle this scenario.