I succeeded in accessing an existing postgresql dbase by using npgsql directly. I used for this:
This bugged me for hours in VS2017. I played with the app.config settings manually with no joy. The steps to fix it for me were:
Npgsql.vsix from the version of npgsql i want to use from hereEntityFramework, Npgsql,
EntityFramework6.Npgsqlapp.config with any references to these packages
including entityFramework and assemblyBindings.EDIT
I also had to add the following to app.config to successfully run Enable-Migrations