Show a WPF window as a modal dialog in XBAP application

   I am developing an XBAP based application (Full trust) in which I need to show a modal dialog. For this, I created a window in xaml and designed my own MessageBox. I expose few APIs like 

MyMessageBox.Show(string message) ;

MyMessageBox.Show(string message, string caption); and so on...

I actually use the API ShowDialog() to show the window (MessageBox). I use this API because it is a blocking call. 

The problem I'm facing right now is when I use this message box in an XBAP application (Full Trust) the messagebox is displayed as a modeless dialog rather than a modal dialog. What exactly is causing this behavior. Am I missing something here? What I mean to say here is while the messagebox is displayed I'm still able to click on items on my page.

Thanks,

Ranjith