I'm trying to convert an Asp.net 2.0 application into an Asp.net 4.0 application. I didn't write the original code and don't have access to the coders. The line of code: return (Thread.CurrentPrincipal as IChatPrinciple) is not working properly. I know that you cannot convert between two derived classes, bu the code worked in .net 2.0. In making the upgrade I'd like to know what change between the frameworks could have caused it to fail. I'd really appreciate the help. Thanks.