could not be found (are you missing a using directive or an assembly reference?)

1. Error    1    The type or namespace name '' could not be found (are you missing a using directive or an assembly reference?) 
   
   
2. PrjForm was set to ".Net Framework 4 Client Profile" I changed it to ".Net Framework 4", and now I have a successful build.
3. http://stackoverflow.com/questions/4764978/the-type-or-namespace-name-could-not-be-found