get_email_from_oauth2_token.cs

public async Task<ActionResult> IndexAsync(string id_token, string state)
{
  if (string.IsNullOrEmpty(id_token) || string.IsNullOrEmpty(state) || !state.Contains(';'))
    return this.BadRequest();

  string platform = state.Split(';')[0];

  if (!Guid.TryParse(state.Split(';')[1], out Guid emailValidationTokenId))
    return this.BadRequest();

  EmailValidationToken emailValidationToken = await this.service.GetByIdAsync(emailValidationTokenId);

  if (emailValidationToken == null)
    return this.BadRequest();

  try
  {
    JwtSecurityToken jwt = new JwtSecurityToken(id_token);

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

  catch
  {
    return this.BadRequest();
  }

  emailValidationToken.Validated = DateTime.Now.ToUniversalTime();
  await this.service.EditAsync(emailValidationToken);

  if (platform == "web")
    return this.Redirect("https://yourapp.com?token=" + emailValidationTokenId);

  // Just shows that everything is OK, and the browser window can be closed
  return this.Redirect("/done");
}