why listbox selected index changed is not firing?

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C#

ASP.NET

Javascript

i have written a javascript method to sortlistbox items it works well in a sence that the item that i type in text box gets highlited but when i click on the highlighted item it dosent gets selected why? the selected indexchanged is not working. here is my javascript code:

function SearchList() {
         var l = document.getElementById("<%= LBox.ClientID %>");
         var tb = document.getElementById("<%= txtDepartments.ClientID %>");
         if (tb.value == "") {
             ClearSelection(l);
         }
         else {
             for (var i = 0; i < l.options.length; i++) {
                 if (l.options[i].value.toLowerCase().match(tb.value.toLowerCase())) {
                     l.options[i].selected = true;

                     return false;
                 }
                 else {
                     ClearSelection(l);
                 }
             }
         }
     }

     function ClearSelection(l) {
         l.selectedIndex = -1;
     }