在 ASP.NET Web API 中，使用 命名空间（namespace） 来作为路由的参数

这个问题来源于我想在 Web API 中使用相同的控制器名称（Controller）在不同的命名空间下，但是 Web API 的默认 路由（Route） 机制是会忽略命名空间的不同的，如果这样做，会看到以下提示：

找到多个与名为“XXX”的控制器匹配的类型。如果为此请求(“{namespace}/{controller}/{action}”)提供服务的路由找到多个控制器，并且这些控制器是使用相同的名称但不同的命名空间定义的(这不受支持)，则会发生这种情况。

在 ASP.NET MVC 中，可以通过建立 区域（Area） 来解决这种问题，但 Web API 并没有区域这种东西。

不过官方早已给出了解决方案，只是并没有作为 Web API 的一部分直接集成。不知道是出于什么考虑。

原文链接：http://blogs.msdn.com/b/webdev/archive/2013/03/08/using-namespaces-to-version-web-apis.aspx

主要思路就是自己重新实现一个可以识别命名空间的路由选择器，然后替换掉系统默认的路由选择器即可。这个命名空间选择器官方也已经帮我们实现，并且提供了完整的项目演示示例。

代码链接：http://aspnet.codeplex.com/SourceControl/changeset/view/dd207952fa86#Samples/WebApi/NamespaceControllerSelector/NamespaceHttpControllerSelector.cs

将此类的实现加入到项目中，并在初始化 Web API 路由时进行替换，在设置路由模板的时候，加入相应的 {namespace} 参数即可：

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        config.MapHttpAttributeRoutes();

        config.Services.Replace(typeof(IHttpControllerSelector), new NamespaceHttpControllerSelector(config));

        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "{namespace}/{controller}/{action}"
        );
    }
}

最后，附上官方 NamespaceHttpControllerSelector 类的实现代码（出处请见上述链接）：

public class NamespaceHttpControllerSelector : IHttpControllerSelector
{
    private const string NamespaceKey = "namespace";
    private const string ControllerKey = "controller";

    private readonly HttpConfiguration _configuration;
    private readonly Lazy<Dictionary<string, HttpControllerDescriptor>> _controllers;
    private readonly HashSet<string> _duplicates;

    public NamespaceHttpControllerSelector(HttpConfiguration config)
    {
        _configuration = config;
        _duplicates = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
        _controllers = new Lazy<Dictionary<string, HttpControllerDescriptor>>(InitializeControllerDictionary);
    }

    private Dictionary<string, HttpControllerDescriptor> InitializeControllerDictionary()
    {
        var dictionary = new Dictionary<string, HttpControllerDescriptor>(StringComparer.OrdinalIgnoreCase);

        // Create a lookup table where key is "namespace.controller". The value of "namespace" is the last
        // segment of the full namespace. For example:
        // MyApplication.Controllers.V1.ProductsController => "V1.Products"
        IAssembliesResolver assembliesResolver = _configuration.Services.GetAssembliesResolver();
        IHttpControllerTypeResolver controllersResolver = _configuration.Services.GetHttpControllerTypeResolver();

        ICollection<Type> controllerTypes = controllersResolver.GetControllerTypes(assembliesResolver);

        foreach (Type t in controllerTypes)
        {
            var segments = t.Namespace.Split(Type.Delimiter);

            // For the dictionary key, strip "Controller" from the end of the type name.
            // This matches the behavior of DefaultHttpControllerSelector.
            var controllerName = t.Name.Remove(t.Name.Length - DefaultHttpControllerSelector.ControllerSuffix.Length);

            var key = String.Format(CultureInfo.InvariantCulture, "{0}.{1}", segments[segments.Length - 1], controllerName);

            // Check for duplicate keys.
            if (dictionary.Keys.Contains(key))
            {
                _duplicates.Add(key);
            }
            else
            {
                dictionary[key] = new HttpControllerDescriptor(_configuration, t.Name, t);
            }
        }

        // Remove any duplicates from the dictionary, because these create ambiguous matches.
        // For example, "Foo.V1.ProductsController" and "Bar.V1.ProductsController" both map to "v1.products".
        foreach (string s in _duplicates)
        {
            dictionary.Remove(s);
        }
        return dictionary;
    }

    // Get a value from the route data, if present.
    private static T GetRouteVariable<T>(IHttpRouteData routeData, string name)
    {
        object result = null;
        if (routeData.Values.TryGetValue(name, out result))
        {
            return (T)result;
        }
        return default(T);
    }

    public HttpControllerDescriptor SelectController(HttpRequestMessage request)
    {
        IHttpRouteData routeData = request.GetRouteData();
        if (routeData == null)
        {
            throw new HttpResponseException(HttpStatusCode.NotFound);
        }

        // Get the namespace and controller variables from the route data.
        string namespaceName = GetRouteVariable<string>(routeData, NamespaceKey);
        if (namespaceName == null)
        {
            throw new HttpResponseException(HttpStatusCode.NotFound);
        }

        string controllerName = GetRouteVariable<string>(routeData, ControllerKey);
        if (controllerName == null)
        {
            throw new HttpResponseException(HttpStatusCode.NotFound);
        }

        // Find a matching controller.
        string key = String.Format(CultureInfo.InvariantCulture, "{0}.{1}", namespaceName, controllerName);

        HttpControllerDescriptor controllerDescriptor;
        if (_controllers.Value.TryGetValue(key, out controllerDescriptor))
        {
            return controllerDescriptor;
        }
        else if (_duplicates.Contains(key))
        {
            throw new HttpResponseException(
                request.CreateErrorResponse(HttpStatusCode.InternalServerError,
                "Multiple controllers were found that match this request."));
        }
        else
        {
            throw new HttpResponseException(HttpStatusCode.NotFound);
        }
    }

    public IDictionary<string, HttpControllerDescriptor> GetControllerMapping()
    {
        return _controllers.Value;
    }
}