get_email_with_google_auth_code.cs

private async Task<string> GetEmailFromGoogleCode(string googleCode)
{
  GoogleAuthorizationCodeFlow flow = new GoogleAuthorizationCodeFlow(
    new GoogleAuthorizationCodeFlow.Initializer
    {
      ClientSecrets = new ClientSecrets()
      {
        ClientId = "XXXXXXXXXX.apps.googleusercontent.com",
        ClientSecret = "XXXXXXXXXX"
      },
      Scopes = new[] { "email" },
    }
  );

  TokenResponse token = await flow.ExchangeCodeForTokenAsync(string.Empty, googleCode, string.Empty, CancellationToken.None);
  JwtSecurityToken jwt = new JwtSecurityToken(token.IdToken);

  return jwt.Claims.First(c => c.Type == ClaimTypes.Email).Value;
}