list box validation

i want to validate for listBox wheth开发者_JAVA技巧er list box is empty of or not i am doing following

 protected void isToListEmpty_ServerValidate(object source, ServerValidateEventArgs args)
    {
        if (toListBox.Items.Count == 0)
        {
            args.IsValid = false;
        }
        else args.IsValid = true;
    }

    <td>
<asp:UpdatePanel ID="second" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:ListBox id="toListBox" runat="server" Width="150px" Height="200" SelectionMode="Multiple">
</asp:ListBox>
<asp:CustomValidator id="isToListEmpty" runat="server" OnServerValidate="isToListEmpty_ServerValidate" ErrorMessage="Select Student Details" ValidationGroup="verify" ControlToValidate="toListBox"></asp:CustomValidator> 

</ContentTemplate>

<Triggers>
<asp:AsyncPostBackTrigger ControlID="ButtonAdd" EventName="Click"></asp:AsyncPostBackTrigger>
<asp:AsyncPostBackTrigger ControlID="ButtonRemove" EventName="Click"></asp:AsyncPostBackTrigger>
<asp:AsyncPostBackTrigger ControlID="ButtonAddAll" EventName="Click"></asp:AsyncPostBackTrigger>
<asp:AsyncPostBackTrigger ControlID="ButtonRemoveAll" EventName="Click"></asp:AsyncPostBackTrigger>
</Triggers>
</asp:UpdatePanel>
</td> 

but it doesn't work for empty list box How to check for this? please let me know

Update: answer rewritten.

The original asker found the following answer (see comments): you must set ValidateEmptyText to true.