新建一个AjaxControlToolkitWebSite 项目,把下面的代码复制到你的页面:

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        <Services>
        <asp:ServiceReference Path="AutoComplete.asmx" />
        </Services>
        </asp:ScriptManager>
        <div>           
            <asp:TextBox runat="server" ID="myTextBox" Width="300" autocomplete="off" />
            <ajaxToolkit:AutoCompleteExtender
                runat="server"
                ID="autoComplete1"
                TargetControlID="myTextBox"
                ServicePath="AutoComplete.asmx"
                ServiceMethod="GetCompletionList"
                MinimumPrefixLength="2"
                CompletionInterval="1000"
                EnableCaching="true"
                CompletionSetCount="12" />
    </div>
    </form>  
</body>
</html> 

AutoComplete.asmx代码:

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

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.Web.Script.Services.ScriptService]
public class AutoComplete : WebService
{
    public AutoComplete()
    {
    }

    [WebMethod]
    public string[] GetCompletionList(string prefixText, int count)
    {
        if (count == 0)
        {
            count = 10;
        }

        Random random = new Random();
        List<string> items = new List<string>(count);
        for (int i = 0; i < count; i++)
        {
            char c1 = (char)random.Next(65, 90);
            char c2 = (char)random.Next(97, 122);
            char c3 = (char)random.Next(97, 122);

            items.Add(prefixText + c1 + c2 + c3);
        }

        return items.ToArray();
    }
}