Hello,
I created a simple test site with themes and resource files and deployed it with the "publish website" option in VS 2005. I've set it up to run under Cassini and IIS (5.1, XP pro box). It renders correctly from IIS but the stylesheets don't seem to be found from cassini. The link tag looks like this:
<link type="text/css" media="all" rel="stylesheet" href="css/StyleSheet.css" /><link href="App_Themes/MPP/StyleSheet.css" type="text/css" rel="stylesheet" /><link href="App_Themes/MPP/StyleSheet2.css" type="text/css" rel="stylesheet" />

Any idea why this might be? The code served to the browser appears to be the same.

The IIS and Cassini sites are indeed pointing to the same folder. If I replace http://localhost:8523/Default.aspx with http://localhost:8523/App_Themes/MPP/StyleSheet.css I get the following exception:

[InvalidCastException: Unable to cast object of type 'System.Web.DefaultHttpHandler' to type 'System.Web.UI.Page'.]
ApplyThemeModule.app_PreRequestHandlerExecute(Object sender, EventArgs e) +40
System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +138
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +65

Going through IIS, http://localhost/published/App_Themes/MPP/StyleSheet.css returns the css file (VisualStudio promptly opens it up).

The site is a small test site I built to test themes and resource files, and precompiled web sites. I'd be happy to send the whole thing to you.

For those interested, the problem was caused by the difference in how HTTP modules (IHttpModule) are handled in IIS and UltiDev Cassini. In our Cassini all modules registered in application's web.config will be called for all requests - not only for ASPX requests like in IIS in its default configuration.

Your IHttpModule code, however, should not assume it always handles only requests for ASPX files. If IIS is configured to send other file types to ASP.NET module, then your IHttpModule implementation will break if it assumes that HttpContext.Current.Handler is always of the System.Web.UI.Page type. While you may get away with such assumption in plain vanilla IIS, fancier configurations of IIS/ASP.NET, or UltiDev Cassini may break your code.

Always check the type of the HttpContext.Current.Handler before casting when you create your own HTTP module.

Best regards,
UltiDev Team.