Ajax三级联动调用aspx后台方法

Test.aspx页面的Html代码

            <div>
                <asp:DropDownList ID="ddlInpatientArea" runat="server">
                </asp:DropDownList>
            </div>
            <div>
                <select name="ddlWard" id="ddlWard"></select>
            </div>
            <div>
                <select name="ddlSickbed" id="ddlSickbed"></select>
            </div>
            <asp:Button Text="text" ID="btAdd" OnClick="btAdd_Click" runat="server" />

Jquery代码

    <script src="Scripts/jquery-1.4.2.min.js" type="text/javascript"></script>
    <script type="text/javascript">
        $(function () {
            var area = $("#ddlInpatientArea");
            var ward = $("#ddlWard");
            var sickbed = $("#ddlSickbed");
            area.bind("change keyup",function () {
                if (area.val().length > 0) {
                    loadWard(area.val());
                    ward.fadeIn("slow");
                    if (ward.val().length > 0) {
                        LoadSickbed(ward.val());
                        sickbed.fadeIn("slow");
                    } else {
                        sickbed.fadeOut("slow");
                    }
                } else {
                    sickbed.fadeOut("slow");
                }
            });

            ward.bind("change keyup",function () {
                if (ward.val().length > 0) {
                    LoadSickbed(ward.val());
                    sickbed.fadeIn("slow");
                } else {
                    sickbed.fadeOut("slow");
                }
            });
            //第一次加载时，先执行一次change事件，初始病房、病床
            area.trigger("change");
        });

        //加载病房
        function loadWard(selectedItem) {
            $.ajax({
                type: "POST",url: window.location + "/LoadWard",//url格式为  要访问的aspx页面/方法名称
                data: "{id: " + selectedItem + "}",contentType: "application/json; charset=utf-8",dataType: "json",async: false,//设置为同步
                success: function (data) {
                    //alert(data.d);
                    showWard(data.d);
                }
            });
        }
        function showWard(data) {
            $("select[name$=ddlWard] > option").remove();
            for (var i = 0; i < data.length; i++) {
                $("select[name$=ddlWard]").append($("<option></option>").val(data[i].ID).html(data[i].WardName));
            }
        }
        //加载病床
        function LoadSickbed(selectedItem) {
            $.ajax({
                type: "POST",url: window.location + "/LoadSickbed",data: "{id: " + selectedItem + "}",success: function (data) {
                    //alert(data.d);
                    showSickbed(data.d);
                }
            });
        }
        function showSickbed(data) {
            $("select[name$=ddlSickbed] > option").remove();
            for (var i = 0; i < data.length; i++) {
                $("select[name$=ddlSickbed]").append($("<option></option>").val(data[i].Sickbedno).html(data[i].Sickbedno));
                alert($("select[name$=ddlSickbed]").html());
            }
        }
    </script>

Test.aspx.cs页面的后台方法

    public partial class Test : System.Web.UI.Page
    {
        protected void Page_Load(object sender,EventArgs e)
        {
            if (!IsPostBack)
            {
                BindInpatientArea();
            }
        }

        #region 绑定病区信息 -void BindInpatientArea()
        /// <summary>
        /// 绑定病区信息
        /// </summary>
        private void BindInpatientArea()
        {
            ddlInpatientArea.DataSource = new ZHBQ.BLL.InpatientArea().GetAll();
            ddlInpatientArea.DataTextField = "InpatientAreaName";
            ddlInpatientArea.DataValueField = "ID";
            ddlInpatientArea.DataBind();
        }
        #endregion

        #region 根据病区Id加载相应病房 +static List<WardInfo> LoadWard(int id)
        /// <summary>
        /// 根据病区Id加载相应病房
        /// </summary>
        /// <param name="id"></param>
        /// <returns></returns>
        [WebMethod]
        public static List<WardInfo> LoadWard(int id)
        {
            ZHBQ.BLL.Ward bll = new Ward();
            List<WardInfo> list = bll.GetWardByAreaId(id);
            return list;
        }
        #endregion

        #region 根据病房Id绑定可用的病床 +static List<SickbedInfo> LoadSickbed(int id)
        /// <summary>
        /// 根据病房Id绑定可用的病床
        /// </summary>
        /// <param name="id"></param>
        /// <returns></returns>
        [WebMethod]
        public static List<SickbedInfo> LoadSickbed(int id)
        {
            ZHBQ.BLL.Sickbed bll = new Sickbed();
            List<SickbedInfo> list = bll.GetAvaiableSkitbedByWardId(id);
            if (list == null)
            {
                list = new List<SickbedInfo>();
                SickbedInfo model = new SickbedInfo();
                model.ID = 0;
                model.Sickbedno = "暂无病床";
                list.Add(model);
            }
            return list;
        }
        #endregion

        protected void btAdd_Click(object sender,EventArgs e)
        {

        }
    }

可能遇到的问题1：

如果html页面中，后两个下拉框使用的是服务器端控件DropDownList，在点击按钮提交时，会出现以下错误：