Force user to select item from autocomplete list

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C#

ASP.NET

Javascript

I want to Force user to select item from autocomplete list.I got the following code from a link.after implementing the below javascript functions the autocomplte list does not appear.If i remove calling the below javascipt functions,the autocomplte list appears.

<script type="text/javascript" language="javascript">
       var isItemSelected = false;
       //Handler for AutoCompleter OnClientItemSelected event
       function onItemSelected() {
           isItemSelected = true;
       }
       //Handler for textbox blur event
       function checkItemSelected(txtPCodebox) {
           if (!isItemSelected) {
               alert("Please select item from the list only!");
               txtPCodebox.focus();
           }
           else {
               //reset isItemSelected
               isItemSelected = false;
           }
       }
   </script>

What is the problem i am not able to figure out.If there is any other method to to the same thing?Please help....