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

原文: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：

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre{ margin: 0em;}
.csharpcode .rem{ color: #008000;}
.csharpcode .kwrd{ color: #0000ff;}
.csharpcode .str{ color: #006080;}
.csharpcode .op{ color: #0000c0;}
.csharpcode .preproc{ color: #cc6633;}
.csharpcode .asp{ background-color: #ffff00;}
.csharpcode .html{ color: #800000;}
.csharpcode .attr{ color: #ff0000;}
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum{ color: #606060;}

[code] using System.ServiceModel;

namespace Artech.AspnetHostingDemo

{

[ServiceContract(Namespace = "urn:artech.com")]

public class CalculatorService

{

[OperationContract]

         public double Add(double x, double y){ return x + y;}

}

}

[/code]

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。

[code] <?xml version="1.0"?>

<configuration>

<system.serviceModel>

<services>

<service name="Artech.AspnetHostingDemo.CalculatorService">

<host>

<baseAddresses>

<add baseAddress="http://127.0.0.1:3721/services"/>

</baseAddresses>

</host>

<endpoint address="calculatorservice" binding="wsHttpBinding" contract="Artech.AspnetHostingDemo.CalculatorService"/>

</service>

</services>

</system.serviceModel>

<system.web>

<compilation debug="true"/>

</system.web>

</configuration>

[/code]

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。

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

[/code]

 

[code] at System.ServiceModel.ServiceHostBase.MakeAbsoluteUri(Uri relativeOrAbsoluteUri, Binding binding, UriSchemeKeyedCollection baseAddresses)

at System.ServiceModel.Description.ConfigLoader.LoadServiceDescription(ServiceHostBase host, ServiceDescription description, ServiceElement serviceElement, Action`1 addBaseAddress)

at System.ServiceModel.ServiceHostBase.LoadConfigurationSectionInternal(ConfigLoader configLoader, ServiceDescription description, ServiceElement serviceSection)

at System.ServiceModel.ServiceHostBase.LoadConfigurationSectionInternal(ConfigLoader configLoader, ServiceDescription description, String configurationName)

at System.ServiceModel.ServiceHostBase.ApplyConfiguration()

at System.ServiceModel.ServiceHostBase.InitializeDescription(UriSchemeKeyedCollection baseAddresses)

at System.ServiceModel.ServiceHost.InitializeDescription(Type serviceType, UriSchemeKeyedCollection baseAddresses)

at System.ServiceModel.ServiceHost..ctor(Type serviceType, Uri[] baseAddresses)

at Artech.AspnetHostingDemo._Default.Page_Load(Object sender, EventArgs e) in e:\WCF Projects\AspnetHostingDemo\AspnetHostingDemo\Default.aspx.cs:line 16

at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e)

at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e)

at System.Web.UI.Control.OnLoad(EventArgs e)   at System.Web.UI.Control.LoadRecursive()

at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

[/code]

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。

[code] public sealed class HostingEnvironment : MarshalByRefObject

{     //其他成员

public static bool IsHosted{ get;}

}

[/code]

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：

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre{ margin: 0em;}
.csharpcode .rem{ color: #008000;}
.csharpcode .kwrd{ color: #0000ff;}
.csharpcode .str{ color: #006080;}
.csharpcode .op{ color: #0000c0;}
.csharpcode .preproc{ color: #cc6633;}
.csharpcode .asp{ background-color: #ffff00;}
.csharpcode .html{ color: #800000;}
.csharpcode .attr{ color: #ff0000;}
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum{ color: #606060;}

[code] public abstract class ServiceHostBase : CommunicationObject, IExtensibleObject<ServiceHostBase>, IDisposable

{

[SecurityTreatAsSafe, SecurityCritical]

private void LoadHostConfig(ServiceElement serviceElement, ServiceHostBase host, Action<Uri> addBaseAddress)

{

HostElement element = serviceElement.Host; if (element != null)

{

if (!ServiceHostingEnvironment.IsHosted)

{                //BaseAddress配置加载与添加

}

}

}

}

[/code]

 

[code] public static class ServiceHostingEnvironment

{

     private static bool isHosted; internal static bool IsHosted{ get{ return isHosted;}}

internal static void EnsureInitialized()

{

if (hostingManager == null)

{

lock (ThisLock)

{

        if (hostingManager == null)

  {

if (!HostingEnvironmentWrapper.IsHosted)

{

                         throw DiagnosticUtility.ExceptionUtility.ThrowHelperError(new InvalidOperationException(SR.GetString("Hosting_ProcessNotExecutingUnderHostedContext", new object[]{ "ServiceHostingEnvironment.EnsureServiceAvailable"})));

}

HostingManager manager = new HostingManager();

HookADUnhandledExceptionEvent();

Thread.MemoryBarrier();

isSimpleApplicationHost = GetIsSimpleApplicationHost();

hostingManager = manager;

isHosted = true;

}

}

}

}

}

[/code]

[code] internal static class HostingEnvironmentWrapper

{

public static bool IsHosted

{

         get{ return HostingEnvironment.IsHosted;}

}

}

[/code]

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：

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre{ margin: 0em;}
.csharpcode .rem{ color: #008000;}
.csharpcode .kwrd{ color: #0000ff;}
.csharpcode .str{ color: #006080;}
.csharpcode .op{ color: #0000c0;}
.csharpcode .preproc{ color: #cc6633;}
.csharpcode .asp{ background-color: #ffff00;}
.csharpcode .html{ color: #800000;}
.csharpcode .attr{ color: #ff0000;}
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum{ color: #606060;}

[code]namespace Artech.AspnetHostingDemo

{

public partial class _Default : System.Web.UI.Page

{

private ServiceHost _serviceHost;

protected void Page_Load(object sender, EventArgs e)

{

this._serviceHost = new ServiceHost(typeof(CalculatorService), new Uri("http://127.0.0.1:3721/services"));

this._serviceHost.Open();

}

}

}

[/code]

另一种方式就是采用绝对地址的方式定义终结点：

[code] <?xml version="1.0"?>

<configuration>

<system.serviceModel>

<services>

<service name="Artech.AspnetHostingDemo.CalculatorService">

<endpoint address="http://127.0.0.1:3721/services/calculatorservice" binding="wsHttpBinding" contract="Artech.AspnetHostingDemo.CalculatorService"/>

</service>

</services>

</system.serviceModel>

<system.web>

<compilation debug="true"/>

</system.web>

</configuration>

[/code]