Keeping ASP.NET Session Open / Alive

[Late to the party...]

Another way to do this without the overhead of an Ajax call or WebService handler is to load a special ASPX page after a given amount of time (i.e., prior to the session state time-out, which is typically 20 minutes):

// Client-side JavaScript
function pingServer() {
    // Force the loading of a keep-alive ASPX page
    var img = new Image(1, 1);
    img.src = '/KeepAlive.aspx';
}

The KeepAlive.aspx page is simply an empty page which does nothing but touch/refresh the Session state:

// KeepAlive.aspx.cs
public partial class KeepSessionAlive: System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        // Refresh the current user session
        Session["refreshTime"] = DateTime.UtcNow;
    }
}

This works by creating an img (image) element and forcing the browser to load its contents from the KeepAlive.aspx page. Loading that page causes the server to touch (update) the Session object, extending the session's expiration sliding time window (typically by another 20 minutes). The actual web page contents are discarded by the browser.

An alternative, and perhaps cleaner, way to do this is to create a new iframe element and load the KeepAlive.aspx page into it. The iframe element is hidden, such as by making it a child element of a hidden div element somewhere on the page.

Activity on the page itself can be detected by intercepting mouse and keyboard actions for the entire page body:

// Called when activity is detected
function activityDetected(evt) {
    ...
}

// Watch for mouse or keyboard activity
function watchForActivity() {
    var opts = { passive: true };
    document.body.addEventListener('mousemove', activityDetected, opts);
    document.body.addEventListener('keydown', activityDetected, opts);
}

I cannot take credit for this idea; see: https://www.codeproject.com/Articles/227382/Alert-Session-Time-out-in-ASP-Net.

I spent a few days trying to figure out how to prolong a users session in WebForms via a popup dialog giving the user the option to renew the session or to allow it to expire. The #1 thing that you need to know is that you don't need any of this fancy 'HttpContext' stuff going on in some of the other answers. All you need is jQuery's $.post(); method. For example, while debugging I used:

$.post("http://localhost:5562/Members/Location/Default.aspx");

and on your live site you would use something like:

$.post("http://mysite/Members/Location/Default.aspx");

It's as easy as that. Furthermore, if you'd like to prompt the user with the option to renew their session do something similar to the following:

    <script type="text/javascript">
    $(function () { 
        var t = 9;
        var prolongBool = false;
        var originURL = document.location.origin;
        var expireTime = <%= FormsAuthentication.Timeout.TotalMinutes %>;

        // Dialog Counter
        var dialogCounter = function() {
            setTimeout( function() {
                $('#tickVar').text(t);
                    t--;
                    if(t <= 0 && prolongBool == false) {
                        var originURL = document.location.origin;
                        window.location.replace(originURL + "/timeout.aspx");
                        return;
                    }
                    else if(t <= 0) {
                        return;
                    }
                    dialogCounter();
            }, 1000);
        }

        var refreshDialogTimer = function() {
            setTimeout(function() { 
                $('#timeoutDialog').dialog('open');
            }, (expireTime * 1000 * 60 - (10 * 1000)) );
        };

        refreshDialogTimer();

        $('#timeoutDialog').dialog({
            title: "Session Expiring!",
            autoOpen: false,
            height: 170,
            width: 350,
            modal: true,
            buttons: {
                'Yes': function () {
                    prolongBool = true;
                    $.post("http://localhost:5562/Members/Location/Default.aspx"); 
                    refreshDialogTimer();
                    $(this).dialog("close");
                },
                Cancel: function () {
                    var originURL = document.location.origin;
                    window.location.replace(originURL + "/timeout.aspx");
                }
            },
            open: function() {
                prolongBool = false;
                $('#tickVar').text(10);
                t = 9;
                dialogCounter();
            }
        }); // end timeoutDialog
    }); //End page load
</script>

Don't forget to add the Dialog to your html:

        <div id="timeoutDialog" class='modal'>
            <form>
                <fieldset>
                    <label for="timeoutDialog">Your session will expire in</label>
                    <label for="timeoutDialog" id="tickVar">10</label>
                    <label for="timeoutDialog">seconds, would you like to renew your session?</label>
                </fieldset>
            </form>
        </div>

In regards to veggerby's solution, if you are trying to implement it on a VB app, be careful trying to run the supplied code through a translator. The following will work:

Imports System.Web
Imports System.Web.Services
Imports System.Web.SessionState

Public Class SessionHeartbeatHttpHandler
    Implements IHttpHandler
    Implements IRequiresSessionState

    ReadOnly Property IsReusable() As Boolean Implements IHttpHandler.IsReusable
        Get
            Return False
        End Get
    End Property

    Sub ProcessRequest(ByVal context As HttpContext) Implements IHttpHandler.ProcessRequest
        context.Session("Heartbeat") = DateTime.Now
    End Sub
End Class

Also, instead of calling like heartbeat() function like:

 setTimeout("heartbeat()", 300000);

Instead, call it like:

 setInterval(function () { heartbeat(); }, 300000);

Number one, setTimeout only fires once whereas setInterval will fire repeatedly. Number two, calling heartbeat() like a string didn't work for me, whereas calling it like an actual function did.

And I can absolutely 100% confirm that this solution will overcome GoDaddy's ridiculous decision to force a 5 minute apppool session in Plesk!