You receive this error when compiling your application:
Error CS0246: The type or namespace name 'XXX' could not be found (are you missing a using directive or an assembly reference?)
This occurs when you have mistyped or misspelled your application’s namespace, generally for a code customization you have added to your application.
Namespaces are case sensitive. Make sure you have used the proper spelling and case for your application’s namespace.
More information on this error is in this Microsoft Knowledge Base article:
http://support.microsoft.com/default.aspx?kbid=304656
Part VII: Troubleshooting Applications
Application Generation and Compilation Error Messages