Does the ListBox SelectedIndexChanged event require postback in ASP.NET?

Question

I'm trying to add a JavaScript function to show all selected items from a ListBox as concatentated strings in a Label on the page. It's needed because AutoPostBack="true" will cause the ListBox to scroll all the way back to the first selected item.

So this code works:

 <script type="text/javascript">
    function Updatelist() { 
        var sel = document.getElementById('<%=lstbxStuff.ClientID%>'); 
        var lbl = document.getElementById('ctl00_cph_lblSelectedStuff');
        var listLength = sel.options.length; 
        var textForListbox = "";
        var list2length = 0;
        for (var i = 0; i < listLength; i++) { 
            if (sel.options[i].selected) { 
               if(list2length == 0) {
                    textForListbox = sel.options[i].text; 
                } else {
                    textForListbox = textForListbox + ", " + sel.options[i].text; 
                }
                list2length++; 
            } 
        } 
        lbl.innerText=textForListbox;

        return textForListbox;
    } 
</script>

Unfortunately I still need the code behind SelectedIndexChanged delegate. Is there a way to use both of these without doing a PostBack? When I set AutoPostBack="false", my delegate never seems to be reached.

Answer 1

I don't think AutoPostBack is the way to go for you if that isn't the behaviour you want. When ASP.Net does a full post back, it is the same as the "traditional" HTML form post, sending the whole content of the form back to ther server and waiting for a response (which happens to be the same page because of how Asp.Net responds). Hence why position in the listbox is lost - it's a brand new listbox you're getting back.

Have you looked at ASP.Net Ajax (UpdatePanels) as one possible option? This will behave the same as a postback in that it'll send data back to the server and call your methods, but only posts back part of the page.

Answer 2

If you want to call a server side deligate then you have to do a PostBack.

What is the code on the server that needs to be ran? You should be able to do all the work in JavaScript, then have a different trigger (not selectedIndexChange) to run the server side code once all the list items are selected.

Have you also seen, Ajax UpdatePanel and maintainScrollPositionOnPostBack="true" so that the page retains it's scroll position after postbacks. However this will only affect the pages scroll bar not the selectbox.