asp.net 控件学习：如何制作带验证的TextBox

[DefaultProperty("Text"), ToolboxData("<{0}:TextBox runat=server></{0}:TextBox>"), Designer("System.Web.UI.Design.WebControls.PreviewControlDesigner, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")]
    public class TextBox : System.Web.UI.WebControls.TextBox
    {
        protected RequiredFieldValidator CanBeBullRFV = new RequiredFieldValidator();

        public TextBox():base()
        {
            this.Attributes.Add("onfocus", "this.className='txt_focus';");
            this.Attributes.Add("onblur", "this.className='txt';");
            base.CssClass = "txt";
        }

        protected override void CreateChildControls()
        {
            if (!CanBeNull)
            {
                CanBeBullRFV.Display = ValidatorDisplay.Dynamic;
                CanBeBullRFV.ControlToValidate = this.ID;       
                CanBeBullRFV.ErrorMessage = "<font color=red>不能为空!</font>";
                this.Controls.AddAt(0, CanBeBullRFV);
            }
        }

        public bool CanBeNull
        {
            get
            {
                return ViewState["CanBeNull"] == null ? true :                                                         Convert.ToBoolean(ViewState["CanBeNull"]);
            }
            set { ViewState["CanBeNull"] = value; }
        }

        protected override void Render(HtmlTextWriter writer)
        {
            base.Render(writer);
            base.RenderChildren(writer);
        }
    }

<%@ Register Assembly="HRM.Control" Namespace="HRM.Control" TagPrefix="cc1" %>


<cc1:TextBox ID="TextBox1" runat="server" CanBeNull="false"></cc1:TextBox>