Return a 401 to the JavaScript when the session has expired

I have an ASP.NET MVC 3 application which uses forms authentication.

Forms authentication has been setup as follows in the web.config.

<authentication mode="Forms">
  <forms loginUrl="Session/LogIn"
         protection="All"
         timeout="60"
         slidingExpiration="true"
         />
</authentication>

When the session expires and I click on something which results in a regular HTTP call, the Session/Login page is shown. Which is all good.

But, when the session expires and I click on something which results in an AJAX ca开发者_JAVA百科ll, the Session/Login page is embedded inside the HTML element which the AJAX call was going to update. i.e. if the AJAX call was to update a div with an id of 123, and because the session expired, then the login form will be placed inside that div.

What I would like to do is to check in the ASP.NET MVC 3 application when the session has expired. If it has, and the call is an AJAX one, then I would like to return no content as the response, and set the return status code to a 401.

In the JavaScript (using jQuery), I can then look for the unauthorised status code of 401, and redirect the user to the Login page. Something like this:

$(document).ajaxError(function(event, response, settings, error) {
    if (response.status == 401) {
        window.document.location = "Login";
    }
});

So what I am looking for is a sure way to check that the session has expired on the C# side, and return an empty response with a status code of 401 which then the JavaScript can handle with cleanly.

in the action that returns the login page (Sesssion/LogIn) check if this is an ajax request,

if(Request.IsAjaxRequest)

and if it is return a Content("<meta http-equiv='refresh' content='0'>"); instead of View()