Question

I have a multi-select <asp:listbox> and a button. I want to disable the button unless >= 1 item in the listbox is selected. How can I do this client-side? I can’t find some sort of OnSelect callback, only OnSelectedIndexChanged which from googling seems complicated to use for this behavior, possibly impossible.

Answer 1

Use OnChange event, and count the selected items :

<script>function enableButton(opt) {
      var selected = new Array();
      var count = 0;
      for (var intLoop=0; intLoop < opt.length; intLoop++) {
         if (opt[intLoop].selected) {
            count++;
         }
      }
      if (count >= 1)
      {
       // disable button
      }
   }
</script>

<select id="list" style="font-size: 11px;"   
    onChange="enableButton(this.options);" MULTIPLE SIZE="5">
    <option value="0">Select...</option>
    <option value="1">1</option>
    <option value="2">2</option>
</select>

EDIT : ASP:ListBox rendered to HTML as select as the code above. So all you need to do is to add the script at the onchange attribute :

myListBox.Attributes.Add("onchange", "enableButton(this.options);");