“无法加载App_Web _ * dll”ASP.NET编译和回收后的异常

> FooService.cs
> FooService.svc
> Default.aspx

文件内容：

[FooService.cs]

using System.ServiceModel;

namespace FooService
{
    [ServiceContract]
    public class FooService
    {
        static FooEngine engine = new FooEngine();

        [OperationContract]
        public string Foo()
        {
            return "bar";
        }
    }

    public class FooEngine
    {

    }
}

[FooService.svc]

<%@ ServiceHost Language="C#" Service="FooService.FooService" %>

[Default.aspx的]

<%@ Page Language="C#" %>
<% var foo = "bar"; %>

我们在Windows Server 2003上使用.Net 4.0(Debug)和IIS6，使用’fooservice’Web和主机文件条目，通过http://fooservice/FooService.svc和default.aspx通过http://fooservice/调用Web服务。此时一切正常。

但是，按照以下步骤，

>在default.aspx中更改代码，保存文件
>加载http://fooservice/(触发ASP.NET编译)
>回收应用程序池

对http://fooservice/FooService.svc的调用失败，并引发以下异常

[FileNotFoundException: Could not load file or assembly 'App_Web_ynlv0b1k,Version=0.0.0.0,Culture=neutral,PublicKeyToken=null' or one of its dependencies. The system cannot find the file specified.]
   System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName,String codeBase,Evidence assemblySecurity,RuntimeAssembly locationHint,StackCrawlMark& stackMark,Boolean throwOnFileNotFound,Boolean forIntrospection,Boolean suppressSecurityChecks) +0
   System.Reflection.RuntimeAssembly.nLoad(AssemblyName fileName,Boolean suppressSecurityChecks) +39
   System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef,Boolean suppressSecurityChecks) +132
   System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString,Boolean forIntrospection) +144
   System.Reflection.Assembly.Load(String assemblyString) +28
   System.ServiceModel.Activation.ServiceHostFactory.CreateServiceHost(String constructorString,Uri[] baseAddresses) +208
   System.ServiceModel.HostingManager.CreateService(String normalizedVirtualPath) +1440
   System.ServiceModel.HostingManager.ActivateService(String normalizedVirtualPath) +44
   System.ServiceModel.HostingManager.EnsureServiceAvailable(String normalizedVirtualPath) +615

[ServiceActivationException: The service '/FooService.svc' cannot be activated due to an exception during compilation.  The exception message is: Could not load file or assembly 'App_Web_ynlv0b1k,PublicKeyToken=null' or one of its dependencies. The system cannot find the file specified..]
   System.Runtime.AsyncResult.End(IAsyncResult result) +679246
   System.ServiceModel.Activation.HostedHttpRequestAsyncResult.End(IAsyncResult result) +190
   System.ServiceModel.Activation.HostedHttpRequestAsyncResult.ExecuteSynchronous(HttpApplication context,String routeServiceVirtualPath,Boolean flowContext,Boolean ensureWFService) +234
   System.ServiceModel.Activation.HttpModule.ProcessRequest(Object sender,EventArgs e) +355
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +148
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step,Boolean& completedSynchronously) +75

这里究竟发生了什么？

备注

>我明白ASP.NET正在编译和加载一个新的App_Web _ *。dll(在第2步)，但为什么它试图加载旧的(已删除)App_Web _ *。dll在循环(3.)之后？
>在FooService.svc中使用完全相同的代码，而不是单独的FooService.cs没有这个问题(？)
删除对FooEngine的静态引用也使得这个问题消失了(？)