Prevent Visual Studio from adding child controls - asp.net

Question

I have a bit of a strange issue here. I created a custom control as such:
public class Textbox : System.Web.WebControls.Placeholder
{
if( [certain criteria are met])
this.Controls.Add(new System.Web.WebControls.Textbox());
}
When working in Visual Studio's designer mode, anytime I add one of my textboxes to the page:
<myControls:Textbox id="txtTest" runat="server" />
Visual studio has a habit of changing my markup to the following:
<myControls:Textbox id="txtTest" runat="server" >
<asp:Textbox runat="server" />
</myControls:Textbox>
This changes the final output markup so that two textboxes appear. Is there a way to prevent visual studio from doing this?
EDIT:
As per request, here is the page markup
<%# Page Language="C#" AutoEventWireup="true" CodeBehind="Test.aspx.cs" Inherits="TestApp.Test" %>
<%# Register Assembly="MyControls.WebControls" Namespace="MyControls.WebControls" TagPrefix="myControls" %>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<table>
<tr>
<td>
<asp:Label ID="lblTest" runat="server">Test Label:</asp:Label>
<myControls:TextBox ID="txtTest" runat="server" Width="80px"></myControls:TextBox>
</td>
</tr>
</table>
</asp:Content>

Answer 1

I think that's happening because your class inherits from the PlaceHolder control, which implements the TemplateControl class. I don't know if there was a reason for this, but it seems like it would be more approriate if you inherited from the TextBox control instead.
EDIT
Since you're using logic to render one of several controls, I would inherit from the WebControl class instead. This will allow you render basically anything you want. Although I must admit, what you're trying to do is probably going to require a substantial amount of work.

Answer 2

If I understand the issue correctly, you are having problems at design time and not runtime.
If this is the case, you should wrap your control add code with a test that ensures it is only executed when you are not in design mode. For example:
if ((!this.InDesignMode) && ( [certain criteria are met]))
this.Controls.Add(new System.Web.WebControls.Textbox());