C# Set timer for viewbag message

I want a message to show/ change after 5 secs are up (I can do this in java script but I need to do it in C#)

I have a function that starts the timer and it works fine. However how can I change the viewbag message after the 5 sec.
Please show exp. code.

public ActionResult Balance()
        {
            var t = "false";

            SetTimer();

            while (exitFlag == true)
            {
                t = "true";
            }

            ViewBag.Response = t;

            return View();
        }

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The Timer function work and it calls the OnTimeEvent ever 5 sec

private void SetTimer()
        {
            // Create a timer with a two second interval.
            System.Timers.Timer aTimer = new System.Timers.Timer(3000);
            // Hook up the Elapsed event for the timer. 
            aTimer.Elapsed += OnTimedEvent;
            aTimer.AutoReset = true;
            aTimer.Enabled = true;
        }

        private void OnTimedEvent(Object source, ElapsedEventArgs e)
        {
            //This does not work.....
            //ViewBag.Response = "true";

            exitFlag = true;
        }

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Thx