ASP.NET UpdatePanel - inline variable - asp.net

Question

I have a server-side variable which is assigned a value in PreRender. For the sake of simplicity, let's imagine I'm doing this:
protected string test;
protected void Page_PreRender(object sender, EventArgs e)
{
test = Guid.NewGuid().ToString();
}
In my markup, the variable is then referred to within an UpdatePanel, like so:
<asp:ScriptManager ID="ScriptManager1" runat="server" />
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:Button ID="Button1" runat="server" Text="Perform postback" />
<script type="text/javascript">
Sys.Application.add_load(function () {
alert('<%=test%>');
});
</script>
</ContentTemplate>
</asp:UpdatePanel>
On first load of the page, the JavaScript alert function gives me a random value, as expected. However, on any subsequent postback, the original value of the Guid is retained.
Can anybody explain why this is so, and offer a solution? I'm fairly sure I can achieve the required behaviour by putting the value of 'test' in a hidden field, for example, but ideally I wouldn't have to sink to those depths. Unfortunately replacing the UpdatePanel with something lighter isn't really an option.

Answer 1

I've found a solution to this, and nor is it quite as dirty as I'd feared. I declare a JS variable outside of the function:
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:Button ID="Button1" runat="server" Text="Perform postback" />
<script type="text/javascript">
var test;
Sys.Application.add_load(function () {
alert(test);
});
</script>
</ContentTemplate>
</asp:UpdatePanel>
In my code-behind, I assign the JS variable a value in the Sys.Application.add_init method, like so:
protected void Page_PreRender(object sender, EventArgs e)
{
ScriptManager.RegisterStartupScript(this, typeof(UpdatePanelTest), "startupScripts", "Sys.Application.add_init(function() {test='" + Guid.NewGuid().ToString() + "';});", true);
}
Voilà, a different Guid is then displayed on first load and every postback.