Validator for list box in asp.net

Question

<asp:ListBox ID="lst_newGrpMembers" 
             DataValueField="Name" 
             SelectionMode="Multiple"
             Width="120px" 
             ToolTip="Press ctrl to select multiple users" 
             runat="server">
</asp:ListBox>

<asp:RequiredFieldValidator ID="RequiredFieldValidator1" 
                            runat="server" 
                            ControlToValidate="lst_newGrpMembers"
                            ErrorMessage="Atleast one member required" 
                            CssClass="Error" 
                            Text="*Atleast one member required"
                            ValidationGroup="CreateGroupValidationGroup">
</asp:RequiredFieldValidator>

I have a required field validator for a list box, it validates and shows error where there is no entry in the list box, but when I add item to the list box, it still shows error. When I select the list box item which I added and click submit it works fine. How to validate the list box.

Solution

On the RequiredFieldValidator, try setting InitialValue to empty string:

<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"          
    InitialValue=""
    ControlToValidate="lst_newGrpMembers" 
    ErrorMessage="Atleast one member required"  
    CssClass="Error"  
    Text="*Atleast one member required" 
    ValidationGroup="CreateGroupValidationGroup"> 
</asp:RequiredFieldValidator>