Silverlight web service call not even hitting servers

I have a custom proxy class for a single WCF web-service (takes a string in, sends a string back). The asynchronous web service call works great from my Windows Form app. However, when I call it from a test Silverlight app I get an error: Hresult 0x80000012. This error is for Extension Attributes on files I believe... Go figure.

Using Fiddler I can confirm that no traffic is making it to IIS which is hosting the service, so it isn't the usual cross-domain restriction issue. I've tried using both "localhost" (which works in the Windows Forms app) and a name defined in my hosts file just in case "localhost" was causing the problem.

I've tested it by making the async call from the UI thread and also from a thread-pool queue item with the same results.

The proxy code is basically just simplified down from what the MS proxy generator creates, setting up bindings and settings programatically instead of via config files, and I use basically the same source to compile normal .Net and Silverlight versions of the libraries involved. (Now, the MS proxy code it is based on is from a WCF web service, not from the RIA template code, so maybe there are differences there...)

Four days of fruitless search on this one. Any help or suggestions would be wildly appreciated!

Figured it out. I was using (stupidly) a backslash in part of the URL that I used to set up the service call. Most of the interior parts of the MS code handled that ok and transposed them to forward slashes -- on Windows Forms everything worked seamlessly in fact. However, the Silverlight libraries couldn't handle it and threw the very helpful "HResult 0x80000012" error.

Changing my backslashes to slashes seems to have fixed the problem!