-----------Default.aspx.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{

}

[System.Web.Services.WebMethodAttribute(), System.Web.Script.Services.ScriptMethodAttribute()]
public static string[] GetCompletionList(string prefixText, int count)
{
if (count == 0) count = 1;
List<string> stringList = new List<string>(count);
for (int i = 0; i < count; i++)
{
stringList.Add(prefixText + i.ToString());
}
return stringList.ToArray();
}
}


-----------Default.aspx-------------


<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>

<%@ Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="cc1" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<link href="StyleSheet.css" rel="stylesheet" type="text/css" />
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>

<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>

</div>


Prova</p>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<cc1:AutoCompleteExtender
ID="TextBox1_AutoCompleteExtender"
runat="server"
Enabled="True"
ServiceMethod="GetCompletionList"
ServicePath=""
TargetControlID="TextBox1"
MinimumPrefixLength="2"
CompletionInterval="1000"
EnableCaching="true"
CompletionSetCount="20"
DelimiterCharacters=";, :"
CompletionListCssClass="autocomplete_completionLis tElement"
CompletionListItemCssClass="autocomplete_listItem"
CompletionListHighlightedItemCssClass="autocomplet e_highlightedListItem"
ShowOnlyCurrentWordInCompletionListItem="true" >
</cc1:AutoCompleteExtender>


Prova2</p>
</form>
</body>
</html>

------------css-------------

.autocomplete_completionListElement
{
overflow : auto;
height : 200px;
border-width : 1px;
border-style : solid;
list-style-type : none;
margin:0px;
padding: 0px;
}
.autocomplete_highlightedListItem
{
background-color: #ffff99;
color: black;
padding: 1px;
}
.autocomplete_listItem
{
background-color : window;
color : windowtext;
padding : 1px;
}