If you are trying to open a connection to an Oracle DB from an asp.net application and it doesn't work (giving you the "ORA-12154" error), but the same connection works perfectly with an udl file or TOAD and tnsping working flawlessy, it's probably a problem of permission to the asp.net user.
Try to give the asp.net user full permission on all the oracle installation dir (and subdirs).
It should work (worked for me)