title: Revoking authentication and redirect to login page after FormsAuthentication have authenticated the user
body: <p>I need to revoke an authentication cookie if the user no longer exists (or some other condition), after the forms authentication mechanism already have received the authentication cookie from the browser and have validated it. I.e. here is the use scenario:</p>
  
  <ol>
  <li>The user have been authenticated, and granted non-expiring auth cookie.</li>
  <li><p>In a few days, the user tries to access my web app again, and as the cookie is valid, the forms authentication mechanism will grant access.</p></li>
  <li><p>Now I want to perform a second check (whatever condition I want), and decide if I want to let the user continue, or to revoke the authentication.</p></li>
  </ol>
  
  <p>The question is - is there an official automated way for this? So far I have come with some possibilities, but I do not know which one is better. I can capture the Authenticate event in global.asax, check whatever I want, and to revoke I clear the cookie, and then one of these:</p>
  
  <ol>
  <li><p>Redirect again to same url - this should work, as this time the forms authentication will fail, and it will redirect to logon page.</p></li>
  <li><p>Throw some exception ??? which one to make the redirect happen w/o me specifying anything?</p></li>
  <li><p>Somehow to get the logon page url from the config file (any ideas how/which config handler to use) and redirect directly?</p></li>
  <li><p>Some FormsAuthentication class/method I have overlooked, which is designed for this?</p></li>
  <li><p>Any other idea?</p></li>
  </ol>