Alert fails in UpdatePanel

I'd like to get an alert from the server inside of an AJAX call in an Update Panel but something is blocking the HttpContext.Current.Response.Write from firing on the client.

Here's a very simple aspx body content

<!-- language: lang-xml -->

<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<div>
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
            <!-- DropDownList doesn't work here -->
        </ContentTemplate>
    </asp:UpdatePanel>
    <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True">
        <asp:ListItem Value="1">First</asp:ListItem>
        <asp:ListItem Value="2">Second</asp:ListItem>
    </asp:DropDownList>
</div>
</form>

And here's where I'm handling it in VB

<!-- language: lang-vb -->

Protected Sub DropDownList1_SelectedIndexChanged(sender As Object, e As EventArgs) _
        Handles DropDownList1.SelectedIndexChanged
    Dim alertMsg As String
    Dim alertScript As String
    'DO OTHER STUFF HERE
    alertMsg = String.Format("You Selected {0}", DropDownList1.SelectedItem.Text)
    alertScript = String.Format("<script type= text/javascript>alert('{0}');</script>", alertMsg)

    System.Web.HttpContext.Current.Response.Write(alertScript)

End Sub

Both times the vb code will fire, but it only scripts an alert message when called outside the the UpdatePanel, instead of inside it.

What am I doing wrong?

You have to register the script using ClientScriptManager since you are using updatepanel. Try the following code. It should work:

Protected Sub DropDownList1_SelectedIndexChanged(sender As Object, e As EventArgs) _
    Handles DropDownList1.SelectedIndexChanged
    Dim alertMsg As String
    Dim alertScript As String
    'DO OTHER STUFF HERE
    alertMsg = String.Format("You Selected {0}", DropDownList1.SelectedItem.Text)
    alertScript = String.Format("<script type= text/javascript>alert('{0}');</script>", alertMsg)

    'register script on startup
    ClientScriptManager.RegisterStartupScript(Me.[GetType](), "Alert", alertScript);
End Sub