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使Web API支持namespace
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问题描述
假设我有一个应用场景：Core Framework可以用于任何区域的站点，其中的CustomersController有个取customer的fullname的方法GetFullName()，可想而知，这个api在中国和美国的站点上，应该得到不同的返回值。如下图所示：
这样的设计可以带来两个好处：
1、利用了OO的思想，可以封装各个区域customer service相关的一些公共逻辑
2、使得client端可以一致的接口访问服务，如：http://hostname/api/customers
这看上去不错，但是为了达到我的目的，就必须让web api支持在不同的namespace（或者area）中，存在相同名称的controller。但是web api默认情况下是不支持的，那么是否可以通过某种方法，使web api支持这种效果呢？答案是肯定的。
查找原因
为了让web api支持namespace（或者area），就必须找到为什么默认情况下web api不支持，在这个过程中，也许能找到切入点。为了能找到原因，我做了如下工作：
1、Server-Side Handlers
从mvc4官网，找到了server端的request处理过程，如下图所示：
从图中我们可以看到，controller是通过HttpControllerDispatcher调度器，来处理的。
2、HttpControllerDispatcher
HttpControllerDispatcher 位于System.Web.Http.Dispatcher命名空间中，其源代码中有一个私有属性：
private IHttpControllerSelector ControllerSelector { get { if (this._controllerSelector == null) { this._controllerSelector = this._configuration.Services.GetHttpControllerSelector(); } return this._controllerSelector; } }
从代码中可以看出，该属性，只是简单的从services容器中，得到IHttpControllerSelector类型的一个对象，所以问题现在转移到在这个controllerselector对象上。
3、DefaultHttpControllerSelector
在this._configuration.Services.GetHttpControllerSelector();这条语句中，_configuration其实就是System.Web.Http.HttpConfiguration，在其构造函数中，可以看到：
this.Services = new DefaultServices(this);
DefaultServices为ServicesContainer的一个子类，所以可以称之为服务容器，定义在System.Web.Http.Services命名空间下，在其构造函数中，有如下代码：
...... this.SetSingle<IHttpControllerActivator>(new DefaultHttpControllerActivator()); this.SetSingle<IHttpControllerSelector>(new DefaultHttpControllerSelector(configuration)); this.SetSingle<IHttpControllerTypeResolver>(new DefaultHttpControllerTypeResolver()); ......
从红色部分这正是我所需要的找的controllerselector。
4、问题所在
从第2步中，如果通过源代码，可以发现：HttpControllerDispatcher 在处理request时，需要通过HttpControllerDescriptor对象的CreateController方法，才能最终实例化一个ApiController。而HttpControllerDescriptor是通过IHttpControllerSelector（默认就是DefaultHttpControllerSelector）的SelectController方法构造的。我进一步在DefaultHttpControllerSelector源码中，发现如下代码：
private ConcurrentDictionary<string, HttpControllerDescriptor> InitializeControllerInfoCache() { ConcurrentDictionary<string, HttpControllerDescriptor> dictionary = new ConcurrentDictionary<string, HttpControllerDescriptor>(StringComparer.OrdinalIgnoreCase); HashSet<string> set = new HashSet<string>(); foreach (KeyValuePair<string, ILookup<string, Type>> pair in this._controllerTypeCache.Cache) { string key = pair.Key; foreach (IGrouping<string, Type> grouping in pair.Value) { foreach (Type type in grouping) { if (dictionary.Keys.Contains(key)) { set.Add(key); break; } dictionary.TryAdd(key, new HttpControllerDescriptor(this._configuration, key, type)); } } } foreach (string str2 in set) { HttpControllerDescriptor descriptor; dictionary.TryRemove(str2, out descriptor); } return dictionary; }
和HttpControllerTypeCache这样一个cache辅助类里的：
private Dictionary<string, ILookup<string, Type>> InitializeCache() { IAssembliesResolver assembliesResolver = this._configuration.Services.GetAssembliesResolver(); return this._configuration.Services.GetHttpControllerTypeResolver().GetControllerTypes(assembliesResolver). GroupBy<Type, string>(t => t.Name.Substring(0, t.Name.Length - DefaultHttpControllerSelector.ControllerSuffix.Length), StringComparer.OrdinalIgnoreCase). ToDictionary<IGrouping<string, Type>, string, ILookup<string, Type>>(g => g.Key, g => g.ToLookup<Type, string>(t => (t.Namespace ?? string.Empty), StringComparer.OrdinalIgnoreCase), StringComparer.OrdinalIgnoreCase); }
这就是问题所在，导致在同一个assembly中，不能有两个相同名字的api controller。否则就会执行：
ICollection<Type> controllerTypes = this._controllerTypeCache.GetControllerTypes(controllerName); if (controllerTypes.Count == 0) { throw new HttpResponseException(request.CreateErrorResponse(HttpStatusCode.NotFound, System.Web.Http.Error.Format(SRResources.ResourceNotFound, new object[] { request.RequestUri }), System.Web.Http.Error.Format(SRResources.DefaultControllerFactory_ControllerNameNotFound, new object[] { controllerName }))); } throw CreateAmbiguousControllerException(request.GetRouteData().Route, controllerName, controllerTypes);
controllerTypes.Count大于0，导致抛出“Multiple types were found that match the controller named…”异常。
解决问题
到现在为止，其实解决办法已经出来了：就是不用上面的两个方法，来构造HttpControllerDispatcher。那么我们就只能自定义IHttpControllerSelector了。所以我自定义了一个NamespaceHttpControllerSelector用于支持namespace，源代码如下：
public class NamespaceHttpControllerSelector : DefaultHttpControllerSelector { private const string NamespaceRouteVariableName = "Namespace"; private readonly HttpConfiguration _configuration; private readonly Lazy<ConcurrentDictionary<string, Type>> _apiControllerCache; public NamespaceHttpControllerSelector(HttpConfiguration configuration) : base(configuration) { _configuration = configuration; _apiControllerCache = new Lazy<ConcurrentDictionary<string, Type>>(new Func<ConcurrentDictionary<string,Type>>(InitializeApiControllerCache)); } private ConcurrentDictionary<string, Type> InitializeApiControllerCache() { IAssembliesResolver assembliesResolver = this._configuration.Services.GetAssembliesResolver(); var types = this._configuration.Services.GetHttpControllerTypeResolver().GetControllerTypes(assembliesResolver).ToDictionary(t => t.FullName, t => t); return new ConcurrentDictionary<string, Type>(types); } public IEnumerable<string> GetControllerFullName(HttpRequestMessage request, string controllerName) { object namespaceName; var data = http://www.mamicode.com/request.GetRouteData();"color: #2b91af">IEnumerable<string> keys = _apiControllerCache.Value.ToDictionary<KeyValuePair<string, Type>, string, Type>(t => t.Key, t => t.Value, StringComparer.CurrentCultureIgnoreCase).Keys.ToList(); if (data.Route.DataTokens == null || !data.Route.DataTokens.TryGetValue(NamespaceRouteVariableName, out namespaceName)) { return from k in keys where k.EndsWith(string.Format(".{0}{1}", controllerName, DefaultHttpControllerSelector.ControllerSuffix), StringComparison.CurrentCultureIgnoreCase) select k; } //get the defined namespace string[] namespaces = (string[])namespaceName; return from n in namespaces join k in keys on string.Format("{0}.{1}{2}", n, controllerName, DefaultHttpControllerSelector.ControllerSuffix).ToLower() equals k.ToLower() select k; } public override HttpControllerDescriptor SelectController(HttpRequestMessage request) { Type type; if (request == null) { throw new ArgumentNullException("request"); } string controllerName = this.GetControllerName(request); if (string.IsNullOrEmpty(controllerName)) { throw new HttpResponseException(request.CreateErrorResponse(HttpStatusCode.NotFound, string.Format("No route providing a controller name was found to match request URI ‘{0}‘", new object[] { request.RequestUri }))); } IEnumerable<string> fullNames = GetControllerFullName(request, controllerName); if (fullNames.Count() == 0) { throw new HttpResponseException(request.CreateErrorResponse(HttpStatusCode.NotFound, string.Format("No route providing a controller name was found to match request URI ‘{0}‘", new object[] { request.RequestUri }))); } if (this._apiControllerCache.Value.TryGetValue(fullNames.First(), out type)) { return new HttpControllerDescriptor(_configuration, controllerName, type); } throw new HttpResponseException(request.CreateErrorResponse(HttpStatusCode.NotFound, string.Format("No route providing a controller name was found to match request URI ‘{0}‘", new object[] { request.RequestUri }))); } }
其实代码并不难懂，核心部分就是：
1、InitializeApiControllerCache方法，用于通过fullname为key，构造一个controller type的一个集合
2、GetControllerFullName方法，从namespace数组和_apiControllerCache集合中，取到符合条件的controller的fullname
到目前为止，我们的工作还剩下最后一步，就是用NamespaceHttpControllerSelector替换DefaultHttpControllerSelector，使其生效。通过以上的分析，其实也很明显了，只需要在Application_Start方法中，用DefaultServices继承下来的Replace即可：
GlobalConfiguration.Configuration.Services.Replace(typeof(IHttpControllerSelector), new NamespaceHttpControllerSelector(GlobalConfiguration.Configuration));
至此，才算大功告成！
参考页面：http://qingqingquege.cnblogs.com/p/5933752.html
使Web API支持namespace