Why aren't client-side (javascript) added controls showing in the postback event?

When adding options to a dropdownlist (or other similar controls) in javascript and posting the page back to the server, the dynamically added options aren't available on the server-side control.

The answer to this question comes from having a good understanding of how the viewstate mechanism works. When the page is first created all controls are serialized to the viewstate which is rendered as a hidden form field. During postback, the page deserializes the viewstate and recreates all controls. So far all the page has done is recreate the controls in their initial state. It must sitll mark the selected values. Todo so it uses the Request.Form collection which, as you hopefully know from ASP (or other), contains only the id and value of selected/checked controls. It loops through Request.Form and for ech entry, finds the corresponding control and marks it as selected/checked.

When you add choices dynamically in javascript, the viewstate has already been rendered and does not contain the information for these items. Therefore, when the page is posted back and the viewstate is used to recreate the controls, the dynamically added ones will not get created. Request.Form can still be used to retrieve the selected/checked values.