asp:listbox validation

I have two asp:ListBox. The ID's are Authors and AuthorsSelected. Authors listbox is loaded with all the authors and AuthorsSelected is empty at first. Using Javascript code, I am moving items from Authors to AuthorsSelected.

Now, before submitting the form, I want to verify that AuthorsSelected listbox is not empty.开发者_JAVA技巧 I tried asp:RequiredFieldValidator and it's not workijng and giving error message.

Please let me know how to validate the AuthorsSelected listbox and make sure it's not empty before submitting the form. Thanks.

Look at this very simple example:

   <p>
    <script language="javascript" type="text/javascript">
        function validateListbox() {
            var listbox = document.getElementById("<%= ListBox1.ClientID  %>");

            if (listbox.length == 0)
                alert("no items!");

            return (listbox.length > 0);

        }        
    </script>


    <asp:ListBox ID="ListBox1" runat="server">
        <asp:ListItem>Item 1</asp:ListItem>
        <asp:ListItem>Item 2</asp:ListItem>
    </asp:ListBox>

</p>
<p>
    <asp:Button ID="BtnSubmit" runat="server" Text="Submit" OnClientClick="return validateListbox()" />
</p>

Please note ListBox1.ClientID usage; I use it in order to reference my listbox in JavaScript.