Oracle data provider for .NET has been installed without machine-wide configuration
I did a lot of stuff which didn't helped. But one thing did.:open visual Studio Command prompt in admin modehttps://docs.microsoft.com/en-us/dotnet/framework/tools/developer-command-prompt-for-vs
there you have to use gacutil /l to list all assemblieshttps://docs.microsoft.com/en-us/dotnet/framework/tools/gacutil-exe-gac-tool
Then delete every entry which includes oracle:
gacutil /u [assembly name]you can get the assembly name out of the list before. Just use the term before the first comma (e.g. Oracle.ManagedDataAccess)
After that I opened Visual Studio and the error message disappeared.What happened was, that in the machine.config were entries which I needed to delete. Which one you have to delete shows the error while you debugg the code.
Hope it will help.