Question

I am looking for an event firing before a DropdownList auto-postback to save Listbox items (populated with JS) in the ViewState.

I tried the OnSelectedIndexChanged event but it’s fired too late.

Can this be done, and how?

Answer 1

This can’t be done, using ViewState. ViewState is set by the server. It can’t be modified by the client in JavaScript.

Suggest one of these options with JavaScript or jQuery:

• modify a hidden input
• modify a cookie value

You can then detect those changes after the page has posted back.

Set up your drop down to modify a hidden element.

$(document).ready(function() {
      $('#<%=DropDownList1.ClientID %>').change(function() { 

         $("#myHiddenInput").val("changed!");

      });
});

....
<input type="hidden" id="myHiddenInput" />

Your DropDownList will post back as per normal.