In this article I will explain how to display Progress Indicator Bar in ASP.Net AJAX Control Toolkit AutoCompleteExtender Control using loading GIF image when it makes call to the server to search the records and hide the image when the records are populated using JavaScript.
Note: If you want to know more about how to use ASP.Net AutoComplete Extender in ASP.Net, you can refer than article using the link below.
HTML Markup
Below is the HTML Markup, which consists of a ToolScriptManager, a TextBox and its Associated ASP.Net AJAX Control Toolkit AutoCompleteExtender control.
<cc1:ToolkitScriptManager ID="ToolkitScriptManager" runat="server">
<asp:TextBox ID="txtContactsSearch" runat="server"></asp:TextBox>
<cc1:AutoCompleteExtender ServiceMethod="SearchCustomers" MinimumPrefixLength="2"
    CompletionInterval="100" EnableCaching="false" CompletionSetCount="10" TargetControlID="txtContactsSearch"
    ID="AutoCompleteExtender1" runat="server" FirstRowSelected="false" OnClientHiding="OnClientCompleted"
    OnClientPopulated="OnClientCompleted" OnClientPopulating="OnClientPopulating">
Above I have added an ASP.Net AJAX AutoCompleteExtender with three client side events OnClientHiding, OnClientPopulated and OnClientPopulating. These events are used to display the loading GIF image when the call is made to search the records and also hides it when the records are populated. The client side methods called on these events are described below.
<script type="text/javascript">
function OnClientPopulating(sender, e) {
    sender._element.className = "loading";
function OnClientCompleted(sender, e) {
    sender._element.className = "";
Client Side Implementation
The OnClientPopulating JavaScript method gets called on the OnClientPopulating event handler and displays the loading GIF image by applying the “loading” CSS class to the TextBox control. While the method OnClientCompleted gets called on the OnClientHiding and OnClientPopulated events and simply removes the CSS class so that the loading GIF image is hidden.
Loading GIF CSS Class
Below is the CSS that you need to add to your page in the HEAD section or CSS class in order to display the loading GIF image when the ASP.Net AJAX Control Toolkit AutoCompleteExtender fetches the records.
<style type="text/css">
    background-image: url(images/loader.gif);
    background-position: right;
    background-repeat: no-repeat;
ASP.Net AJAX AutoCompleteExtender with Progress Indicator Bar | Display Animated Loading GIF Image in ASP.Net AJAX AutoCompleteExtender