Custom Textbox content

[DavyQuyo]

Hey everyone

I made a litle custom textbox.
Something pretty easy to do but I encountered a problam which I cann't figure out how to do it.

on this moment I made this litle script

[ParseChildren(false)]
    [ValidationProperty("Text")]
    [DefaultProperty("Text")]
    [ToolboxData("<{0}:TextBox runat=\"server\"></{0}:TextBox>")]
    [ToolboxBitmap(typeof(System.Web.UI.WebControls.TextBox))]
    public class TextBox : WebControl, IPostBackDataHandler
    {
        public event EventHandler TextChanged;

        protected virtual void OnTextChanged(EventArgs e)
        {
            if (TextChanged != null)
                TextChanged(this, e);
        }

        public String Text
        {
            get { return (String)ViewState["Text"]; }
            set { ViewState["Text"] = value; }
        }

        protected override HtmlTextWriterTag TagKey
        {
            get
            {
                return HtmlTextWriterTag.Input;
            }
        }

        protected override void AddAttributesToRender(HtmlTextWriter writer)
        {
            writer.AddAttribute(HtmlTextWriterAttribute.Name, this.UniqueID);
            writer.AddAttribute(HtmlTextWriterAttribute.Type, "text");
            writer.AddAttribute(HtmlTextWriterAttribute.Value, Text);
            writer.AddAttribute(HtmlTextWriterAttribute.Id, this.ClientID);
            //base.AddAttributesToRender(writer);
        }

        protected override void RenderContents(HtmlTextWriter writer)
        {
            //base.RenderContents(writer);
        }
        
        protected override void AddParsedSubObject(object obj)
        {
            if (obj is LiteralControl)
                Text = ((LiteralControl)obj).Text;
            else
                throw new System.Web.HttpException("You cannot have a child control of type " + obj.GetType().Name.ToString());
        }
        
        public virtual bool LoadPostData(string postDataKey, NameValueCollection postCollection)
        {
            string presentValue = Text;
            string postedValue = postCollection[postDataKey];

            if (presentValue == null || !presentValue.Equals(postedValue))
            {
                Text = postedValue;
                return true;
            }

            return false;
        }

        public virtual void RaisePostDataChangedEvent()
        {
            OnTextChanged(EventArgs.Empty);
        }

Offcourse the textbox works perfect but I have a problem in design mode.
When you add the textbox to your page you get this

<custum:TextBox runat="server" ID="TextBox1">Test]</custom:Textbox>

now the problem I have is between the TextBox tags of the HTML in designmode
if you press < you can see you can add all kind of controls between the the tags but in my script I disabled all controls except text. I want to ask now how to disable any controls between the tags in designmode