<cc1:AutoCompleteExtender ID="txtLB_AutoCompleteExtender" runat="server"  DelimiterCharacters="|" Enabled="True"
MinimumPrefixLength="1" ServiceMethod="GetTextString" ServicePath="WebService.asmx" TargetControlID="txtLB" UseContextKey="True"
CompletionSetCount="30" CompletionInterval="100" >
</cc1:AutoCompleteExtender >

上面属性：TargetControlID就是文本框的ID，CompletionInterval是调用间隔，1000是一秒

ServiceMethod是调用的WebService里的方法 GetTextString

ServicePath是WebService的路径，所以要添加WebService

3.

    /// <summary>
    ///WebService 的摘要说明
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    //若要允许使用 ASP.NET AJAX 从脚本中调用此 Web 服务，请取消对下行的注释。
    [System.Web.Script.Services.ScriptService] //不要遗忘掉了取消注释
    public class WebService : System.Web.Services.WebService
    {
        GradeSettingLoader _loader = new GradeSettingLoader();
        public WebService()
        {

            //如果使用设计的组件，请取消注释以下行
            //InitializeComponent();
        }
        private string[] autoCompleteWordList = null;//保存获取的内容
        [WebMethod]
        public string[] GetTextString(string prefixText, int count)
        //string prefixText, int count 两个参数必须原封不动照写，包括大小写也是一样 返回参数只能是字符串数组
        //prefixText表示用户输入的前缀，count表示返回的个数
        {
            if (string.IsNullOrEmpty(prefixText) == true)
                return null;
            if (autoCompleteWordList == null)
            {
                DataSet ds = _loader.GetGradeLB();
                string[] temp = new string[ds.Tables[0].Rows.Count];//临时数组
                int i = 0;
                foreach (DataRow dr in ds.Tables[0].Rows)
                {
                    temp[i] = Convert.ToDecimal(dr["INSURANCEGRADE"]).ToString();
                    i++;
                }
                Array.Sort(temp, new CaseInsensitiveComparer());
                autoCompleteWordList = temp;//临时数组赋值给刚刚前面申明的数组

            }
            //定位二叉树起点
            int index = Array.BinarySearch(autoCompleteWordList, prefixText, new CaseInsensitiveComparer());
            if (index < 0)
            {
                index = ~index;//修复起点
            }
            //搜索符合条件的数据
            int matchCount = 0;
            for (matchCount = 0; matchCount < count && matchCount + index < autoCompleteWordList.Length; matchCount++)
            {
                if (autoCompleteWordList[index + matchCount].StartsWith(prefixText, StringComparison.CurrentCultureIgnoreCase) == false)
                {
                    break;
                }
            }
            //处理搜索结果
            string[] matchResultList = new string[matchCount];
            if (matchCount > 0)
            {
                Array.Copy(autoCompleteWordList, index, matchResultList, 0, matchCount);
            }
            return matchResultList;
        }

}

4.注意，WebService里方法返回的是Json数据，即已逗号分隔的，如果数据里本来就有逗号，就比较麻烦了，可以用特殊字符“?“替换常规逗号","。