PollingDuplex doesn't work in IIS Express

I have a Silverlight 4 chat program that uses a WCF service with pollingDuplexHttpBinding.

The program works OK In:
* Cassini
* IIS Local Web Server
* All non-Duplex WCF Services work fine in any setting

But it does NOT work in IIS 7.5 Express. A call is made but a 500 HTTP error is given, and nothing happens (no exceptions are thrown, program just ignores it)

Here's my Web.Config

Code:

---

<configuration>
<system.web>

    <compilation debug="true" strict="false" explicit="true" targetFramework="4.0" />
</system.web>
<system.webServer>
    <defaultDocument enabled="true">
        <files>
            <add value ="DuelPortalOnlineTestPage.html"/>
            <add value="ClientBin/DuelPortalOnline.xap"/>
        </files>
    </defaultDocument>
</system.webServer>
<system.serviceModel>
    <services>
        <service name="DuelPortalOnline.Web.ChatService" >
            <host>
                <baseAddresses>
                </baseAddresses>
            </host>
            <endpoint address="" binding="pollingDuplex" bindingConfiguration="myPollingDuplex"
                contract="DuelPortalOnline.Web.ChatService"  behaviorConfiguration="duplexBehavior" />
            <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange" />
        </service>
    </services>
    <extensions>
        <bindingExtensions>
            <!-- This is added when VS doesn't recognize the dll reference right away
            <add name="pollingDuplex" type="System.ServiceModel.Configuration.PollingDuplexHttpBindingCollectionElement,System.ServiceModel.PollingDuplex, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
            -->
        </bindingExtensions>
    </extensions>
    <behaviors>
        <serviceBehaviors>
            <behavior name="duplexBehavior">
                <serviceMetadata httpGetEnabled="true"  />
                <serviceDebug includeExceptionDetailInFaults="true" />   
            </behavior>
        </serviceBehaviors>
    </behaviors>
    <bindings>
        <pollingDuplex>
            <binding name="myPollingDuplex" duplexMode="MultipleMessagesPerPoll" >
            </binding>
        </pollingDuplex>
    </bindings>
    <serviceHostingEnvironment aspNetCompatibilityEnabled="true"
        multipleSiteBindingsEnabled="true" />
</system.serviceModel>

---

Does anyone have any ideas? After some testing I know it's not a firewall issue. What's so different about iis local and iis express?

You could try to adjust the .config so the full error message is revealed, that might help

I have never use pollingDuplex, but my I ask why you are using it? It seems harder to use than it is worth.

Since it is a chat application, when the server receives a message the server should push to the clients, which you can accomplish with simple duplex calls. Just define a callback on the service contract and make the operational contract that should update the clients one way. Then when you add a service reference to the client, it will automatically create the callback interface that you will have to flesh out.

With this you have a place for the service to "talk" to the client and you can also define events so when the callback gets a message, you raise an event the client is subscribed to.

If you REALLY want to use the polling, do as lighting suggests and get more detail on the error. Add a Trace to the config file and look it over.

Code:

---

<system.diagnostics>
    <sources>
      <source name="System.ServiceModel" switchValue="Error" propagateActivity="true">
        <listeners>
          <add name="ServiceTraceListener" type="System.Diagnostics.XmlWriterTraceListener" initializeData="C:\Temp\MyTrace.log" />
        </listeners>
      </source>
    </sources>
  </system.diagnostics>

---