public class MyTextBox : TextBox
    {
        private RequiredFieldValidator req;

        public MyTextBox()
        {
            
        }

        protected override void OnInit(EventArgs e)
        {
            req = new RequiredFieldValidator();
            req.ID = String.Format("req{0}", this.ID);
            req.ControlToValidate = this.ID;
            req.EnableClientScript = false;
            req.Display = ValidatorDisplay.Dynamic;

            RequiredID = req.ID;

            req.ErrorMessage = MyErrorMessage;
            this.Controls.Add(req);
        }

        protected override void Render(System.Web.UI.HtmlTextWriter writer)
        {
            if (this.ReadOnly)
            {
                writer.Write(@"<span class=""{0}"">{1}</span>", this.Text);
            }
            else
            {
                base.Render(writer);
                req.RenderControl(writer);
            }
        }

        public string MyErrorMessage
        {
            get { return ((string)ViewState["MyErrorMessage"]); }
            set { ViewState["MyErrorMessage"] = value; }
        }