Selecting partial text from a textbox in code behind

Hello,

I want to be able to select a part of text in a textbox. As I undersatnd it this is diffrent from what I'm use to in desktop apps. For example I load the textbox with some text. The user adds more information to the textbox so I want to save the portion that the user entered.

I realize I could just have two textboxes one for the old and one for the new but I hate the look of that design. A cool alternative maybe to popup and textbox ontop of the old one but I have no idea how to accomplish that. I thought I could place one textbox overtop the other and just hide until it is needed but the designer (VS2008) wo'nt allow me to place one control ontop of another.

Thanks, for any help.

Ty
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