WCF技术剖析之六：为什么在基于ASP.NET应用寄宿（Hosting）下配置的BaseAddress无效

蒋金楠

发布于 2018-01-16 06:40:24

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本篇文章来源于几天前一个朋友向我咨询的问题。问题是这样的，他说他采用ASP.NET应用程序的方式对定义的WCF服务进行寄宿（Hosting），并使用配置的方式对服务的BaseAddress进行了设置，但是在创建ServiceHost的时候却抛出InvalidOperationException，并提示相应Address Scheme的BaseAddress找不到。我意识到这可能和WCF中用于判断服务寄宿方式的逻辑有关，于是我让这位朋友将相同的服务寄宿代码和配置迁移到GUI程序或者Console应用中，看看是否正常。结果如我所想，一切正常，个人觉得这应该是WCF的一个Bug。今天撰文与大家讨论，看看大家对这个问题有何见解。

一、问题重现

问题很容易重现，假设我们通过ASP.NET应用对服务CalculatorService进行寄宿，为了简单起见，我将服务契约和服务实现定义在一起。CalculatorService的定义如下面的代码片断所示：

   1: using System.ServiceModel;
   2: namespace Artech.AspnetHostingDemo
   3: {
   4:     [ServiceContract(Namespace = "urn:artech.com")]
   5:     public class CalculatorService
   6:     {
   7:         [OperationContract]
   8:         public double Add(double x, double y) { return x + y; }
   9:     }
  10: }

下面是服务寄宿相关的配置，在<host>/<baseAddresses>配置节中为服务添加了一个Scheme为http的BaseAddress：http://127.0.0.1:3721/services，那么终结点的地址就可以定义为基于该BaseAddress的相对地址了：calculatorservice。

   1: <?xml version="1.0"?>
   2: <configuration>
   3:   <system.serviceModel>
   4:     <services>
   5:       <service name="Artech.AspnetHostingDemo.CalculatorService">
   6:         <host>
   7:           <baseAddresses>
   8:             <add baseAddress="http://127.0.0.1:3721/services"/>
   9:           </baseAddresses>
  10:         </host>
  11:         <endpoint address="calculatorservice" binding="wsHttpBinding" contract="Artech.AspnetHostingDemo.CalculatorService"/>
  12:       </service>
  13:     </services>
  14:   </system.serviceModel>
  15:   <system.web>
  16:     <compilation debug="true"/>
  17:   </system.web>
  18: </configuration>

我们把服务寄宿的代码定义在一个Web Page的Load事件中。但程序执行到到创建ServiceHost的时候，抛出如下图所示的InvalidOperationException异常。

下面是错误信息和异常的StackTrace:

   1: Could not find a base address that matches scheme http for the endpoint with binding WSHttpBinding. Registered base address schemes are [].

   1: at System.ServiceModel.ServiceHostBase.MakeAbsoluteUri(Uri relativeOrAbsoluteUri, Binding binding, UriSchemeKeyedCollection baseAddresses)   
   2: at System.ServiceModel.Description.ConfigLoader.LoadServiceDescription(ServiceHostBase host, ServiceDescription description, ServiceElement serviceElement, Action`1 addBaseAddress)   
   3: at System.ServiceModel.ServiceHostBase.LoadConfigurationSectionInternal(ConfigLoader configLoader, ServiceDescription description, ServiceElement serviceSection)   
   4: at System.ServiceModel.ServiceHostBase.LoadConfigurationSectionInternal(ConfigLoader configLoader, ServiceDescription description, String configurationName)   
   5: at System.ServiceModel.ServiceHostBase.ApplyConfiguration()   
   6: at System.ServiceModel.ServiceHostBase.InitializeDescription(UriSchemeKeyedCollection baseAddresses)   
   7: at System.ServiceModel.ServiceHost.InitializeDescription(Type serviceType, UriSchemeKeyedCollection baseAddresses)   
   8: at System.ServiceModel.ServiceHost..ctor(Type serviceType, Uri[] baseAddresses)   
   9: at Artech.AspnetHostingDemo._Default.Page_Load(Object sender, EventArgs e) in e:\WCF Projects\AspnetHostingDemo\AspnetHostingDemo\Default.aspx.cs:line 16   
  10: at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e)   
  11: at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e)   
  12: at System.Web.UI.Control.OnLoad(EventArgs e)   at System.Web.UI.Control.LoadRecursive()   
  13: at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

二、问题分析

通过上面提供StackTrace，我们可以看到错误发生在WCF试图将BaseAddress和RelativeAddress进行组合生成AbsoluteAddress的时候。从错误消息可以看出，在进行地址的组合时，由于没有找到适合绑定类型（WsHttpBinding）Scheme（http）的BaseAddress，导致了异常的抛出。

要解答这个问题，首先要解释一下WCF的BaseAddress在不同服务寄宿（Service Hosting）方式下的定义方式。对于WCF服务的自我寄宿（Self Hosting）或者采用Windows Service进行服务寄宿，我们可以通过代码或者形如上面的配置为服务指定一系列的BaseAddress（对于一个既定的URI Scheme，只能由唯一的BaseAddress）。但是对于采用IIS或者WAS进行服务寄宿，我们需要为相应的服务定义一个.svc文件，我们通过访问.svc文件的方式来调用相应的服务。对于后者，.svc文件得地址就是WCF服务的BaseAddress，所以WCF会忽略BaseAddress的配置。

那么WCF采用怎样的方式来判断当前服务寄宿的方式是基于IIS呢，还是其他呢？答案是通过System.Web.Hosting.HostingEnvironment的静态属性IsHosted。对于ASP.NET有一定了解的人应该很清楚，在一个ASP.NET应用下，该属性永远返回为True。也就是说，WCF会把基于ASP.NET应用的服务寄宿，看成是基于IIS的服务寄宿，这显然是不对的。

   1: public sealed class HostingEnvironment : MarshalByRefObject
   2: {     //其他成员
   3:     public static bool IsHosted { get; }
   4: }

WCF对BaseAddress配置的加载和添加的逻辑定义在ServiceHostBase的LoadHostConfig方法中，大致的逻辑如下面的代码所示：

   1: public abstract class ServiceHostBase : CommunicationObject, IExtensibleObject<ServiceHostBase>, IDisposable
   2: {
   3:     [SecurityTreatAsSafe, SecurityCritical]
   4:     private void LoadHostConfig(ServiceElement serviceElement, ServiceHostBase host, Action<Uri> addBaseAddress)
   5:     {
   6:         HostElement element = serviceElement.Host; if (element != null)
   7:         {
   8:             if (!ServiceHostingEnvironment.IsHosted)
   9:             {                //BaseAddress配置加载与添加
  10:             }
  11:         }
  12:     }
  13: }

   1: public static class ServiceHostingEnvironment
   2: {
   3:     private static bool isHosted; internal static bool IsHosted { get { return isHosted; } }
   4:     internal static void EnsureInitialized()
   5:     {
   6:         if (hostingManager == null)
   7:         {
   8:             lock (ThisLock)
   9:             {
  10:                 if (hostingManager == null)
  11:                 {
  12:                     if (!HostingEnvironmentWrapper.IsHosted)
  13:                     {
  14:                         throw DiagnosticUtility.ExceptionUtility.ThrowHelperError(new InvalidOperationException(SR.GetString("Hosting_ProcessNotExecutingUnderHostedContext", new object[] { "ServiceHostingEnvironment.EnsureServiceAvailable" })));
  15:                     }
  16:                     HostingManager manager = new HostingManager();
  17:                     HookADUnhandledExceptionEvent(); 
  18:                     Thread.MemoryBarrier(); 
  19:                     isSimpleApplicationHost = GetIsSimpleApplicationHost();
  20:                     hostingManager = manager; 
  21:                     isHosted = true;
  22:                 }
  23:             }
  24:         }
  25:     }
  26: }
  27:

  1: internal static class HostingEnvironmentWrapper
   2: {
   3:     public static bool IsHosted
   4:     {
   5:         get { return HostingEnvironment.IsHosted; }
   6:     }
   7: }

三、解决方式

其实这种情况也没有什么好的解决方案，不外乎就是避免通过配置的方式设置服务的BaseAddress，可以通过代码的方式来设置。如下面的代码所示：

   1: namespace Artech.AspnetHostingDemo
   2: {
   3:     public partial class _Default : System.Web.UI.Page
   4:     {
   5:         private ServiceHost _serviceHost;
   6:         protected void Page_Load(object sender, EventArgs e)
   7:         {
   8:             this._serviceHost = new ServiceHost(typeof(CalculatorService), new Uri("http://127.0.0.1:3721/services")); 
   9:             this._serviceHost.Open();
  10:         }
  11:     }
  12: }

另一种方式就是采用绝对地址的方式定义终结点：

   1: <?xml version="1.0"?>
   2: <configuration>
   3:   <system.serviceModel>
   4:     <services>
   5:       <service name="Artech.AspnetHostingDemo.CalculatorService">
   6:         <endpoint address="http://127.0.0.1:3721/services/calculatorservice" binding="wsHttpBinding" contract="Artech.AspnetHostingDemo.CalculatorService"/>
   7:       </service>
   8:     </services>
   9:   </system.serviceModel>
  10:   <system.web>
  11:     <compilation debug="true"/>
  12:   </system.web>
  13: </configuration>