Using the System.Configuration.ConfigurationSettings.AppSettings["whatever"] and getting the obsolete error message?
While the definition of "obsolete" might be arguable, it still works - but of course you should fix it. Change to
System.Configuration.ConfigurationManager. AppSettings["whatever"]
If you are getting an error on that one - add a reference to System.Configuration.dll, cause appearantly Microsoft have moved the AppSettings class out of the System dll.