<asp:TextBox ID="tbString" runat="server" CssClass="regform" Width="199px"></asp:TextBox>
     <asp:MaskedEditExtender ID="MEEtbString" runat="server" TargetControlID="tbString"
               Enabled="False" MessageValidatorTip="true" OnFocusCssClass="MaskedEditFocus"
               OnInvalidCssClass="MaskedEditError" ErrorTooltipEnabled="True" PromptCharacter="_"
               AutoComplete="False" />

        public int MaxLength
        {
            get { return tbString.MaxLength; }
            set { tbString.MaxLength = (value > 2000) ? 2000 : value;}
        }
        public string SMask
        {
            set
            {
                if (System.String.IsNullOrEmpty(value)) {MEEtbString.Enabled = false;}
                else { MEEtbString.Mask = value.ToString(); MEEtbString.Enabled = true;}
            }
            get {return MEEtbString.Mask;}
        }