Viewbag is null in alert - asp.net-mvc

Question

i wrote behind code.
but Viewbag.message will show null in alert message .myvar is a variable.
i used breakpoint , myvar will set by Viewbag.message correctly. but it will be shown null in alert .
<script src="~/Scripts/jquery-1.7.1.js"></script>
<script src="~/Scripts/jquery.unobtrusive-ajax.js"></script>
#{string myvar = ViewBag.AlertMessage;}
#using (Ajax.BeginForm("action", "controller", new AjaxOptions { HttpMethod = "Post", OnSuccess = "Messsage" }))
{
<script type="text/javascript">
function Messsage() {
alert('#ViewBag.AlertMessage'); //infact, it shows alert('');
}
</script>

Answer 1

<script type="text/javascript">
function Messsage() {
alert('#myvar'); // should show "hello there"
}
</script>
#using (Ajax.BeginForm("AjaxAction", "Home", new AjaxOptions { HttpMethod = "Post", OnSuccess = "Messsage" }))
{
<script type="text/javascript">
function Messsage() {
alert("#ViewBag.AjaxMessage");
}
</script>
<input type="submit" value="Submit" />
}

Answer 2

is the ViewBag.AlertMessage being defined in the controller action ?
you can replace the data by
#{string myvar = "hello there !";}
<script type="text/javascript">
function Messsage() {
alert('#myvar'); // should show "hello there"
}
</script>
or define your viewbag item in the action method behind the result
public ActionResult Index() {
ViewBag.AlertMessage = "hello there !"
return View();
}

Answer 3

Try setting the value for ViewBag.AlertMessage in controller action that returns the view on which you have defined the Ajax.Begin form.
eg
public class HomeController : Controller
{
public ActionResult Index()
{
ViewBag.Message = "Modify this template to jump-start your ASP.NET MVC application.";
ViewBag.AlertMessage = "AjaxMessage.";
return View();
}
}
on Index view i have placed the following code, calling AjaxAction on Home Controller.
#using (Ajax.BeginForm("AjaxAction", "Home", new AjaxOptions { HttpMethod = "Post", OnSuccess = "Messsage" }))
{
<script type="text/javascript">
function Messsage() {
alert("#ViewBag.AjaxMessage");
}
</script>
<input type="submit" value="Submit" />
}