Giving Focus to a textbox

hi there
i have a list box and it contains about 50 textboxes arranged in a tabular fashion.
the textboxes are represented by an array myTextBox[50].

what i wanted to do is to focus a particular textbox in the listbox.
so what i did is

myTextBox[3].Focus()

to select the fourth textbox on the listbox.
but it didnt work…the textbox was not focused…
please help
and i have set the tabnaviagtion of textbox to Local too…
thanks