asp.net - jquery validation is not working properly -

i new jquery.

by googling have done following code not working.

i have txtlogin textbox , btnlogin button.

i trying simple thing. if textbox expty error message should displayed in lblloginerror.

following code:

 $(document).ready(function () {          $('#btnlogin').click(function () {              var loginid = $("#txtlogin").val();              if ($("#txtlogin").val() == "") {                   $("#lblloginerror").text("please enter login id");                   return false;              }              else {                  $("#lblloginerror").text("");                  return true;              }           });      }); 

my problem is>> first time when textbox empty , click on button gives me error message on lable[its working fine according code].

second time when enters thing in textbox , press button removing error message.[this fine according code].

but third time when removes entered text , presses button , not shows me error message.

*note: using .net in ajax update panel. page not gets refreshed each time. can problem?

it useful link jquery validation

example:

<script>   $(document).ready(function(){    $("#commentform").validate(); }); </script>  <form id="commentform">   <input type="text" id="txtlogin" class="required"/>   <input class="submit" type="submit" value="submit"/> </form> 

here can apply class required.