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.

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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.