Entity Framework with Oracle.ManagedDataAccess: Error 175 in EDMX File
I had this same error message on my new laptop. I'm running Visual Studio 2019 (v16.4.5). The project containing my model-first .edmx files was a .NET 4.6 class library with references to Oracle.ManagedDataAccess.dll v12.1.24160719 and Oracle.ManagedDataAccess.EntityFramework.dll v12.1.2400.
The fix was to download and install the Oracle Developer Tools for Visual Studio 2019. This .vsix file can be downloaded here:https://www.oracle.com/database/technologies/dotnet-odtvsix-vs2019-downloads.html
Now I can work with my .edmx files in the designer just like I could on my old laptop.