You have to register the script using [ClientScriptManager][1] 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


  [1]: http://msdn.microsoft.com/en-us/library/system.web.ui.clientscriptmanager.registerstartupscript%28v=vs.100%29.aspx

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



    <form id="form1" runat="server">
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <div>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                
            </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



    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?