title: Asp.net core - Two-factor authentication login process
body: <p>I could apply two-factor authentication for user in my asp.net core app, but in the login phase, I have a problem with this logic:</p>
  
  <ul>
  <li>if a user disables 2FA feature -> Directly login.</li>
  <li>if a user enables 2FA feature -> Show 2FA input field.</li>
  </ul>
  
  <p>I know that it's impossible to check whether users disable or enable 2FA before logging in. Then how to check this after a user enters correct username and password but still unauthorized if user enables 2FA?  </p>