difficulty in using asp.net controls in webpart

[silentsoldier]

i initially added one TextBox and a Button in a webpart programmatically and
on Button_Click event i captured the text of TextBox , this task was done
successfully.

Then i added a ListBox and a Button, now on button_Click i am trying to
capture selected item in ListBox through ListBox.SelectedValue or
ListBox.SelectedItem.Value none of it is working. i tried almost everything
but unable to capture selected value on postback when button is clicked.

I repeated the listbox process by replacing ListBox with DropDownList but
had no effect.

So if any one come up with solution do let me know