Dropdownlist post back problem in user control

[jonathanyan]

Hi all:
I am dynamically creating two dropdownlist controls in a user
control, the data of the second dropdownlist control depends on the
selected value of the first dropdownlist control, the first
dropdownlist control has a post back event, in which it will bind the
data to the second control based on the selected value of the first
control.
Both auto postback and data binding work fine, but when I tried
selected a value in the second dropdownlist control and use a button
to commit the change, the selected value returned for the second
control is always the first element.
I have also tried to define a post-back event for the second
control, and I got the same result.

Any suggestion will be appreciated!

Jonathan