Ecco il codice....

------------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">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>


AutoCompleteextender



</div>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<cc1:AutoCompleteExtender ID="TextBox1_AutoCompleteExtender" runat="server"
DelimiterCharacters="" Enabled="True" ServiceMethod="GetCompletionList"
ServicePath="" TargetControlID="TextBox1" UseContextKey="True">
</cc1:AutoCompleteExtender>
</form>
</body>
</html>


-----------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, string contextKey)
{
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();
}
}