Given a valid Heroku potgresql connection string this code will create the expected connection string to be used inside .net core 2.0 applications.

Install the package from here: https://www.nuget.org/packages/HerokuPGParser/

Inside your .net core 2.0 Startup.cs class in ConfigureServices method add:

var pgConn = Environment.GetEnvironmentVariable("DefaultConnection");

if (!string.IsNullOrWhiteSpace(pgConn))
    conStr = HerokuPGParser.ConnectionHelper.BuildExpectedConnectionString(pgConn);

services.AddDbContext<AppDb>(options => options.UseNpgsql(conStr));

Where AppDb is your EF Code DbContext.

Have fun!