Can't access datagrid textbox values when visible=false

Hi,

I have a datagrid with a number of textboxes on it.
I want one column of my table to be visible=false because it contains
the id of the record from the table.
For some reason when I set visible=true on the aspx page I can access
it no problem, but when I set visible=false the value that is returned
is always equal to ""

Dim testing8 As String = CType(e.Item.Cells(8).Controls(1),
TextBox).Text
Dim testing9 As String = CType(e.Item.Cells(9).Controls(1),
TextBox).Text

these are two lines from my code. They are identical textboxes,
except that one has visible=true and on has visible=false.

The visible=true one returns the correct value, and the visible=false
one returns "".

Any suggestions would be much appreciated.

Cheers,

Andrew