i am adding dynamic control (textbox) and i set the property to visible = false
but i dont find the control in the tree collection, i want to hide it from user se
I believe that putting Visible="false" onto a control with stop it rendering to the browser, so no, you won't find it.
Instead, you can try using an HtmlInputHidden control, which will generate a hidden input field.
Hidden fields aren't a great way of doing things since they're able to be modified fairly easily on the client side using various intercept tools.