WCF技術剖析之二十七: 如何將一個服務發布成WSDL[基於HTTP-GET的實現]

（提供模擬程序）

基於HTTP-GET的元數據發布方式與基於WS-MEX原理類似,但是ServiceMetadataBehavior需要做的更多額外的工作。原因很簡單，由於在WS-MEX模式下，我們為寄宿的服務添加了相應的MEX終結點，那麼當服務被成功寄宿後，WCF已經為元數據的消息交換建立了如圖1所示的分發體系，我們需要做的僅僅是對MEX終結點的DispatchRuntime進行相應的定制而已。

圖1 WCF服務端分發體系

但是如果采用HTTP-GET模式，實際上我們需要從ChannelDispatcher開始，重新構建整個分發體系。接下來，我們在《WS-MEX原理》提供實例的基礎上，對我們自定義ServiceMetadataBehaviorAttribute進行進一步的完善，使之同時對兩種模式的元數據發布提供支持。 （Source Code從這裡下載）

首先，我們需要定義一個新的服務契約接口：IHttpGetMetadata，Get操作處理任何形式的消息請求，因為它的輸入參數和返回類型均為Message，並且Action和ReplyAction為*。

   1: using System.ServiceModel;
   2: using System.ServiceModel.Channels;
   3: namespace ServiceMetadataBehaviorSimulator
   4: {
   5:     [ServiceContract(Name = "IHttpGetMetadata", Namespace = "http://www.artech.com/")]
   6:     public interface IHttpGetMetadata
   7:     {
   8:         [OperationContract(Action = "*", ReplyAction = "*")]
   9:         Message Get(Message msg);
  10:     }
  11: }

然後我們讓前面定義的MetadataProvisionService實現IHttpGetMetadata接口，在這裡無需再寫任何多余的代碼，因為MetadataProvisionService已經具有了一個Get方法。

   1: public class MetadataProvisionService : IMetadataProvisionService, IHttpGetMetadata
   2: {
   3:       //省略成員
   4: }

接下來的工作就是構建一個全新的ChannelDispatcher，以及關聯EndpointDispatcher，最後對EndpointDispatcher的DispatchRuntime進行定制。為此，我單獨寫了一個方法：CreateHttpGetChannelDispatcher。

   1: [AttributeUsage(AttributeTargets.Class)]
   2: public class ServiceMetadataBehaviorAttribute : Attribute, IServiceBehavior
   3: {
   4:     //其他成員
   5: private const string SingletonInstanceContextProviderType = "System.ServiceModel.Dispatcher.SingletonInstanceContextProvider,System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089";
   6:      private const string SyncMethodInvokerType = "System.ServiceModel.Dispatcher.SyncMethodInvoker,System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089";
   7:      private const string MessageOperationFormatterType = "System.ServiceModel.Dispatcher.MessageOperationFormatter, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089";
   8:
   9:     private static void CreateHttpGetChannelDispatcher(ServiceHostBase host, Uri listenUri, MetadataSet metadata)
  10:     {
  11:         //創建Binding
  12:         TextMessageEncodingBindingElement messageEncodingElement = new TextMessageEncodingBindingElement() { MessageVersion = MessageVersion.None };
  13:         HttpTransportBindingElement transportElement = new HttpTransportBindingElement();
  14:         Utility.SetPropertyValue(transportElement, "Method", "GET");
  15:         Binding binding = new CustomBinding(messageEncodingElement, transportElement);
  16:
  17:         //創建ChannelListener
  18:         IChannelListener listener = binding.BuildChannelListener<IReplyChannel>(listenUri, string.Empty, ListenUriMode.Explicit, new BindingParameterCollection());
  19:         ChannelDispatcher dispatcher = new ChannelDispatcher(listener, "ServiceMetadataBehaviorHttpGetBinding", binding) { MessageVersion = binding.MessageVersion };
  20:
  21:         //創建EndpointDispatcher
  22:         EndpointDispatcher endpoint = new EndpointDispatcher(new EndpointAddress(listenUri), "IHttpGetMetadata", "http://www.artech.com/");
  23:
  24:         //創建DispatchOperation，並設置DispatchMessageFormatter和OperationInvoker
  25:         DispatchOperation operation = new DispatchOperation(endpoint.DispatchRuntime, "Get", "*", "*");
  26:         operation.Formatter = Utility.CreateInstance<IDispatchMessageFormatter>(MessageOperationFormatterType, Type.EmptyTypes, new object[0]);
  27:         MethodInfo method = typeof(IHttpGetMetadata).GetMethod("Get");
  28:         operation.Invoker = Utility.CreateInstance<IOperationInvoker>(SyncMethodInvokerType, new Type[] { typeof(MethodInfo) }, new object[] { method });
  29:         endpoint.DispatchRuntime.Operations.Add(operation);
  30:
  31:         //設置SingletonInstanceContext和InstanceContextProvider
  32:         MetadataProvisionService serviceInstance = new MetadataProvisionService(metadata);
  33:         endpoint.DispatchRuntime.SingletonInstanceContext = new InstanceContext(host, serviceInstance);
  34:         endpoint.DispatchRuntime.InstanceContextProvider = Utility.CreateInstance<IInstanceContextProvider>(SingletonInstanceContextProviderType, new Type[] { typeof(DispatchRuntime) }, new object[] { endpoint.DispatchRuntime });
  35:         dispatcher.Endpoints.Add(endpoint);
  36:
  37:         //設置ContractFilter和AddressFilter
  38:         endpoint.ContractFilter = new MatchAllMessageFilter();
  39:         endpoint.AddressFilter = new MatchAllMessageFilter();
  40:
  41:         host.ChannelDispatchers.Add(dispatcher);
  42:     }
  43: }

大體上介紹一下創建ChannelDispatcher的邏輯。首先創建綁定對象，該綁定由兩個綁定元素構成：TextMessageEncodingBindingElement和HttpTransportBindingElement，這些因為元數據請求消息就是單純的HTTP-GET請求消息，並不是一個SOAP，所以需要將HttpTransportBindingElement的消息版本設為None，並將Method屬性（這是一個internal屬性）設為GET。

然後利用創建的綁定對象創建ChannelListener，並基於該ChannelListener創建ChannelDispatcher對象。當ChannelDispatcher成功創建，開始創建EndpointDispatcher對象，並定制該EndpointDispatcher的DispatchRuntime。這其中包括創建DispatchOperation對象以及相關的消息格式化器以及操作執行器。然後是我們熟悉的對InstanceContextProvider和SingletonInstanceContext的設定。最後需要設置EndpointDispatcher的兩個消息篩選器：契約篩選器和地址篩選器，在這將它們設置成MatchAllMessageFilter類型，使之能夠匹配所有的請求消息。關於WCF的消息篩選機制，在《WCF技術剖析（卷1）》第2章有詳細介紹。

待DispatchRuntime被成功定制，將創建的EndpointDispatcher添加到ChannelDispatcher的EndpointDispatcher列表，最終再將ChannelDispatcher添加到ServiceHost的ChannelDispatcher列表中。

然後，我們在ServiceMetadataBehaviorAttribute添加兩個屬性：HttpGetEnabled和HttpGetUrl，前者表示是否采用基於HTTP-GET的元數據發布模式，後者指定元數據發布的地址。並將上面定義的CreateHttpGetChannelDispatcher添加到ApplyDispatchBehavior方法中。

   1: [AttributeUsage(AttributeTargets.Class)]
   2: public class ServiceMetadataBehaviorAttribute : Attribute, IServiceBehavior
   3: {
   4:     //其他成員
   5:     public bool HttpGetEnabled
   6:     { get; set; }
   7:     public string HttpGetUrl
   8:     { get; set; }
   9:     public void ApplyDispatchBehavior(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)
  10:     {
  11:         MetadataSet metadata = GetExportedMetadata(serviceDescription);
  12:         CustomizeMexEndpoints(serviceDescription, serviceHostBase, metadata);
  13:         if (this.HttpGetEnabled)
  14:         {
  15:             CreateHttpGetChannelDispatcher(serviceHostBase, new Uri(this.HttpGetUrl), metadata);
  16:         }
  17: }
  18: }

那麼現在我們就可以通過下面的方式將ServiceMetadataBehaviorAttribute應用到我們的CalculatorService，並通過HttpGetUrl屬性指定原數據發布的目標地址：

   1: [ServiceMetadataBehavior(HttpGetEnabled = true, HttpGetUrl = "http://127.0.0.1:9999/calculatorservice/mex")]
   2: public class CalculatorService : ICalculator, IMetadataProvisionService
   3: {
   4:    //省略成員
   5: }

如果CalculatorService被成功寄宿，直接通過浏覽器訪問元數據發布的地址（http://127.0.0.1:9999/calculatorservice/mex），你可以看到與圖2一樣的結果。

圖2 通過IE獲取發布的元數據