"Could not load App_Web_*.dll" Exception after ASP.NET compile & recycle

Question

I recently came across code of the following structure:

• FooService.cs
• FooService.svc
• Default.aspx

Content of the files:

[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]

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

We're using .Net 4.0 (Debug) and IIS6 on Windows Server 2003 with a 'fooservice' web and a hostfile entry to call the web service via http://fooservice/FooService.svc and default.aspx via http://fooservice/. Everything works perfectly at this point.

However, after the following steps,

1. Change code in default.aspx, save file
2. Load http://fooservice/ (triggers ASP.NET compile)
3. Recycle app pool

a call to http://fooservice/FooService.svc fails and throws the following exception

[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, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +39
   System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection, Boolean suppressSecurityChecks) +132
   System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, 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, Version=0.0.0.0, Culture=neutral, 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

What exactly is going on here?

Remarks

• I understand that ASP.NET is compiling and loading a new App_Web_*.dll (during step 2), but why is it trying to load the old (deleted) App_Web_*.dll after the recycle (3.)?
• using the exact same code inline in FooService.svc instead of a separate FooService.cs does not have this problem (?)
• removing the static reference to the FooEngine also makes this problem go away (?)