Listview webpart - get selected items

Hi All,

I have created a custom visual webpart in SP 2010 with following code. "SP.ListOperation.Selection.getSelectedItems" is not returning the selected items

<script type="text/javascript">

 function ShowSelectedItems() {
  var ctx = SP.ClientContext.get_current();
  var selectedItems = SP.ListOperation.Selection.getSelectedItems(ctx);
  var key;
  for (key in selectedItems) {
   alert(selectedItems[key].id);
  }


 }</script><SharePoint:ListView ID="caseView" runat="server" ListId="fc368824-cf03-49cf-a545-bf7a3dde8326" ViewId="34b4c31f-a3bb-4f1b-9834-4c3419bfed1b" /><input name="btnShowSelectedItems" id="btnShowSelectedItems" onclick="ShowSelectedItems();" type="button" value="Show Selected Items"/>