A few days ago a coworker asked me about implementing an autocomplete textbox with not only text items, but also images. I thought, how hard can it be? I am sure the guys that made the AutoCompleteExtender in the AjaxControlToolkit thought about it. Yeah, right!

So, I needed to tap into the list showing mechanism of the AutoCompleteExtender, then (maybe) into the item selected mechanism. The AutoCompleteExtender exposes the OnClientShowing and the OnClientItemSelected properties. They expect a function name that accepts a behaviour and an args parameters.

Ok, the extender creates an html element to contain the list completion items or gets one from the property CompletionListElementID (which is obsoleted anyway). It creates a LI element for each item (or a DIV in case of setting CompletionListElementID). So all I had to do was iterate through the childNodes of the container element and change their content.

Then, on item selected, unfortunately the AutoCompleteExtender tries to take the text value with firstChild.nodeValue, which pretty much fails if the first child of the item element is not a text node. So we will tap in OnClientItemSelected, which args object contains item, the text extracted as mentioned above (useless to us), and the object that was passed from the web service that provided the completion list. The last one we need, but keep reading on.

So the display is easy (after you get the hang of the Microsoft patterns). But now you have to return a list of objects, not mere strings, in order to get all the information we need, like the text and the image URL. Here is the piece of code that interprets the values received from the web service:
// Get the text/value for the item
try {
var pair = Sys.Serialization.JavaScriptSerializer.deserialize('(' + completionItems[i] + ')');
if (pair && pair.First) {
// Use the text and value pair returned from the web service
text = pair.First;
value = pair.Second;
} else {
// If the web service only returned a regular string, use it for
// both the text and the value
text = pair;
value = pair;
} catch (ex) {
text = completionItems[i];
value = completionItems[i];

In other words, it first tries to deserialize the string received, then it checks if it is a Pair object (if it has a First property) else it passes the object as value and text! If deserialization fails, the entire original string is considered. Bingo! So on the server side we need to serialize the array of strings we want to send to the client. And we do that by using System.Web.Script.Serialization.JavaScriptSerializer. You will see how it goes into the code.

So far we displayed what we wanted, we sent what we wanted, all we need is to set how we want the completion items to appear. And for that I could have used a simple string property, but I wanted all the goodness of the intellisense in Visual Studio and all the objects I want, without having to Render them manually into strings.

So, the final version of the AutoCompleteExtender with images is this: A class that inherits AutoCompleteExtender, but also INamingContainer. It has a property ItemTemplate of the type ITemplate which will hold the template we want in the item. You also need a web service that will use the JavascriptSerializer to construct the strings returned.

Here is the complete code:

using System;
using System.IO;
using System.Web.UI;
using System.Web.UI.WebControls;
using AjaxControlToolkit;

namespace Siderite.Web.WebControls
/// <summary>
/// AutoCompleteExtender with templating
/// </summary>
public class AdvancedAutoComplete : AutoCompleteExtender, INamingContainer
private ITemplate _template;

public ITemplate ItemTemplate
get { return _template; }
set { _template = value; }

protected override void OnInit(EventArgs e)
const string script = @"
function AdvancedItemDisplay(behaviour,args) {
var template=behaviour.get_element().getAttribute('_template');
//if (!template==null) template='${0}';
for (var i=0; i<behaviour._completionListElement.childNodes.length; i++) {
var item=behaviour._completionListElement.childNodes[i];
var vals = item._value;
var html=template;
for (var c=0; c<vals.length; c++)
html=html.replace(new RegExp('\\$\\{'+c+'\\}','g'),vals[c]);

function AdvancedSetText(behaviour,args) {
var vals=args._value;
var element=behaviour.get_element();
var control = element.control;
if (control && control.set_text)
element.value = vals[0];
ScriptManager.RegisterClientScriptBlock(this, GetType(), "AdvancedAutoComplete", script, true);

OnClientShowing = "AdvancedItemDisplay";
OnClientItemSelected = "AdvancedSetText";

protected override void OnPreRender(EventArgs e)
string template = GetTemplate();
((TextBox)TargetControl).Attributes["_template"] = template;

private string GetTemplate()
Content ph=new Content();
ph.Page = Page;
HtmlTextWriter htw = new HtmlTextWriter(new StringWriter());
return htw.InnerWriter.ToString();


using System.Collections.Generic;
using System.Web.Script.Serialization;
using System.Web.Script.Services;
using System.Web.Services;

/// <summary>
/// Web service to send auto complete items to the AdvancedAutoComplete extender
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class MainService : WebService
public string[] GetCompletionList(string prefixText, int count)
JavaScriptSerializer jss=new JavaScriptSerializer();
List<string> list=new List<string>();
for (int c = 0; (list.Count< count) && (c < 100000); c++)
string s = (c*c).ToString();
if (s.StartsWith(prefixText))
object[] item = new object[] {s, "images/demo.gif"};
return list.ToArray();

An example of the use

<asp:TextBox runat="server" ID="tbMain"></asp:TextBox>
<cc1:AdvancedAutoComplete ID="aceMain" runat="server" BehaviorID="mhMain" TargetControlID="tbMain"
ServiceMethod="GetCompletionList" ServicePath="~/MainService.asmx" MinimumPrefixLength="0"
<asp:Image runat="server" ID="imgTest" ImageUrl="${1}" /><asp:Label runat="server"
ID="lbTest" Text="${0}"></asp:Label>

That's it! All you have to do is make sure the controls in the template render ${N} text that gets replaced with the first, second, Nth item in the list sent by the web service. The text that will be changed in the textbox is always the first item in the list (${0}).

Restrictions: if you want to use this a control in a library and THEN add some more functionality on the Showing and ItemSelected events, you need to take into account that those are not real events, but javascript functions, and the design of the autocompleteextender only accepts one function name. You could create your own function that also call on the one described here, but that's besides the point of this blog entry.



You register it as any control in ASP.Net: http://haacked.com/archive/2006/11/14/register_custom_controls_in_web.config.aspx As for the Javascript, I can&#39;t help you. I haven&#39;t been working in that area for quite some time.


Anibal Díaz

Hi!, how you register this control on the page aspx? and, where I can implement the javascript and events from advancedautocomplete? thanks!

Anibal Díaz

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