CustomValidator验证的使用方法

<asp:TextBox ID="txtNum" runat="server" Width="400px" ></asp:TextBox>

<asp:CustomValidator ID="cvNum" runat="server" ErrorMessage="此登记证号已存在！请勿重复添加！" onservervalidate="cvNum_ServerValidate" ControlToValidate="txtNum"  SetFocusOnError="True" Display="Dynamic"></asp:CustomValidator>

protected void cvNum_ServerValidate(object source, ServerValidateEventArgs args)
{
     DataTable dt = DB.GetDataTable("i_id", "tab_dengji", "dengjihao='" + txtNum.Text.Trim() + "'", "");
    args.IsValid = false;
    if (Request.QueryString["id"] != null)
    args.IsValid = true;
   else
   {
       if (dt != null && dt.Rows.Count > 0)
      args.IsValid = false;
      else
      args.IsValid = true;
   }
}