Use Required Field Validator for CheckboxList

<script language="javascript" type="text/javascript">
    function ValidateChkList(source, arguments)
    {                                                   
        arguments.IsValid = IsCheckBoxChecked() ? true : false;  

    }

    function IsCheckBoxChecked()
    {   
        var isChecked = false;
        var list =document.getElementById('<%= CheckBoxList1.ClientID %>');
        if(list != null)
        {
         for (var i=0; i<list.rows.length; i++)
         {
          for (var j=0; j<list.rows[i].cells.length; j++)
          {
           var listControl = list.rows[i].cells[j].childNodes[0];                      
           if(listControl.checked)
           {                      
            isChecked = true;
           }
          }
         }
         }
         return isChecked;

    }
    </script>

   <div>
        <asp:CheckBoxList ID="CheckBoxList1" runat="server">
            <asp:ListItem Text="C#" ></asp:ListItem>
            <asp:ListItem Text="VB"></asp:ListItem>
        </asp:CheckBoxList>
        <asp:CustomValidator ID="CustomValidator1" ClientValidationFunction="ValidateChkList"
            runat="server" >Required.</asp:CustomValidator>
    </div>
    <div>
        <asp:Button ID="Button2" runat="server" Text="Submit" OnClick="Button2_Click"  />
    </div>