Currently I'm working on a project where we just recently upgraded our version of Oracle from 11g to 12c on our testing server, as well as my local development setup.
After the upgrade on my local setup, I upgraded my ODAC (primarily for Oracle.DataAccess.dll) to use the 12c 32-bit version. It took a bit of trial and error, along with uninstalling and reinstalling the ODAC before I was at last able to reconnect with my DB.
We are now looking at updating the testing server's ODAC. I performed the installation yesterday and like with my local, I was unable to connect with the DB.
I have placed the tnsnames.ora file into it's proper place, and this file is just a copy of the original. I have used a test program to find out if I can even open a connection, which I can using the new Oracle.DataAccess.dll. However, when I try to import that over to the main site, we cannot connect. This comes even after I delete the original reference, add the new one in pointing to the location of the Oracle.DataAccess.dll.
A coworker of mine mentioned that it doesn't matter what version of the ODAC we have (11g or 12c), that the 11g version should be able to connect with the 12c DB still with no issue. I kind of question this since he has made such statements in the past on other issues, along with the fact he agreed we needed to upgrade the ODAC initially.
Is this a true statement? If not, are there any steps I can take to resolve the issue short of uninstalling and reinstalling again? It shouldn't take that kind of effort, lol.
UPDATE: I have confirmed that my coworker was correct, using the 11g ODAC on a 12c Oracle DB does work. However, lol, he uninstalled it before I could talk with him further about it, so yeah...
I am still finding a similar issue even after removing 11g's ODAC. My coworker also cleaned up the Registry entries that had some 11g references, but still, we are having issues.
UPDATE (8/8/2017): The issue has been at long last figured out. The 12c ODAC I had installed was not compatible with the version of Visual Studio I was using on the server (VS 2012). After looking through the requirements for the ODAC, I found that out and rolled my eyes upon that realization. We uninstalled the 12c ODAC and reinstalled the 11g ODAC. Everything works normally now.
I have asked that VS be upgraded, just so in the future we do have a compatible version to work with. Yeah, that is unlikely to happen though, lol...
The answer to the issue is compatibility between VS 2012 and 12c ODAC. I found out after rereading the specs for the 12c ODAC that the version of Visual Studio on our server is not covered. We have VS 2012, which the ODAC doesn't go back to (at least for our Release version of ODAC), hence the issue.
After finding this, we went back to the 11g ODAC, which still works with a 12c DB.