Silverlight Selecting text with doubleclick

Question

In a standard text box, say ASP.Ne if you have say a order number, something along the lines of this => 177-1111

if you double click on any of the first 3 chars '177' then '177' is selected, and you can then Copy that using Ctrl+C

if you double click on any of the '1111' chars, it selects only 1111.

In Silverlight, using a textbox, and readonly set. when I do this I get '177-'

anyone know (a) what I have done wrong :) or (b) some other way to do it?

thanks for any help you can provide.