asp.net实现省市联动并且保存状态，实现原理：初次使用C#传统的DataBind进行数据绑定，选择省，市后进行ajax方式加载下级列表，并将选定值保存到Hidden中，点击提交后根据Hidden的值进行省，市绑定，再将Hidden的值赋给DropDownList的SelectedValue，从而实现状态的保存。
HTML代码
<asp:HiddenField ID="HiddenFieldShiAreaId" runat="server" />
<asp:HiddenField ID="HiddenFieldQuAreaId" runat="server" />
<asp:HiddenField ID="HiddenAreaId" runat="server" />
<span class="searchDivP">学生省市：</span>
<asp:DropDownList ID="DropDownListSheng" runat="server">
</asp:DropDownList>
<asp:DropDownList ID="DropDownListShi" runat="server">
</asp:DropDownList>
<asp:DropDownList ID="DropDownListQu" runat="server">
</asp:DropDownList>
javascript代码（此处使用jQuery）
$(document).ready(function() {
$("#DropDownListSheng").change(function() {
var buxianstr = "<option selected=\"selected\" value=\"0\">不限</option>";
loadArea("DropDownListShi", this.value);
});
$("#DropDownListShi").change(function() {
var areaid = this.value;
loadArea("DropDownListQu", this.value);
$("#HiddenFieldShiAreaId").val(areaid);
});
$("#DropDownListQu").change(function() {
$("#HiddenFieldQuAreaId").val(this.value);
});
});
function loadArea(objid, areaid) {
$.ajax({
type: "POST",
url: "../Friend/Friend.aspx?action=getshi",
dataType: "json",
data: "areaid=" + areaid,
success: function(msg) {
var shiHtml = "";
var selectstr = "";
shiHtml += "<op
