Question

On the front-end, I’ve initialised FB and login using:

  window.fbAsyncInit = function () {
        FB.init({
            appId: @facebookAppId, // App ID
            status: true, // check login status
            cookie: true, // enable cookies to allow the server to access the session
            xfbml: true,  // parse XFBML
            oauth: true
        });

        // Additional initialization code here
        FB.Event.subscribe('auth.login', function () {
            window.location = "/facebookPostAuth.aspx";
        });

        FB.Event.subscribe('auth.logout', function (response) {
            window.location = '/logout';
        });

        // Hack to fix http://bugs.developers.facebook.net/show_bug.cgi?id=20168 for IE7/8/9
        FB.UIServer.setLoadedNode = function (a, b) { FB.UIServer._loadedNodes[a.id] = b; };

    };

    // Load the SDK Asynchronously
    (function (d) {
        var js, id = 'facebook-jssdk'; if (d.getElementById(id)) { return; }
        js = d.createElement('script'); js.id = id; js.async = true;
        js.src = "//connect.facebook.net/en_US/all.js";
        d.getElementsByTagName('head')[0].appendChild(js);
    } (document));

On the back-end, how do I check if there is a login session existing using Facebook C# SDK?

This is as much as I’ve got, but I’m not sure how to pass in the APP ID / Secret to get more information (such as email which I’ve inluded in login scope):

 var client = new FacebookClient();
            dynamic me = client.Get("me");
            string firstName = me.first_name;
            string lastName = me.last_name;
            string email = me.email;

Answer 1

var client = new FacebookClient(appID, appSecret);

or

var client = new FacebookClient(accessToken);

You can grab the access token client-side with the javascript SDK and pass it up to the server in a hidden form field (conventional or ajax), or the C# SDK exposes what it finds in the cookie and/or signed request via FacebookWebContext.Current.AccessToken. In the past I have found some timing issues where Facebook did not update the cookie with a new access token/signed request in a timely fashion, which is why I get it in Javascript and post it to the server.