Save multiple texbox insert in one string

HI,

I have webform with listbox, texbox and   button1 and button

When I type text int texbox and press button1 I add item in listbox

for example I do that five times and then I have 5 item in listbox

on button2 click I wont to insert all that item(text value) in one string

I do that for saving it in to database as one text

I try something with List<> but it dosent work

also I try  to set all item in listbox as selected (whit foreach loop) but it gives me text only for first row