I know this is an old topic, BUT… Google has changed something on their security settings.
Was struggling with all the answers until I checked my email with a mail from Google stating that “we’ve recently blocked a sign-in attempt on your Google account”.
That led me to this page:
Google Account Security
Under the “Access for less secure apps” section, you can enable access to your account from other devices/applications… like your C# application.
Note, there is no longer an “application specific” section.
Hope this helps someone… I just lost 30 minutes of my life…