求asp.net从后台获取数据的三级联动的ajax代码

下拉列表的内容从数据库获取，方法写好了。但是ajax不会写。求个代码

AJAX省市县3级联动

function getXmlHttp() {
var xmlhttp;
if (window.XMLHttpRequest) {
// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp = new XMLHttpRequest();
} else {// code for IE6, IE5
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
return xmlhttp;
}

function getBuildings(){
    var campus_id = $('#campusSelect option:selected') .val();
    $("#buildingSelect").empty();
    $("#roomSelect").empty();
    if (typeof(campus_id) == "undefined" || campus_id == "") {
        alert("请选择校区！")
        return;
    }
    var xmlhttp = getXmlHttp();
    xmlhttp.onreadystatechange=function()
      {
      if (xmlhttp.readyState==4 && xmlhttp.status==200)
        {
            var select = document.getElementById("buildingSelect");
            var buildings = $.parseJSON(xmlhttp.responseText);
            for(var i = 0; i < buildings.length; i++) {
                select.options.add(new Option(buildings[i].building_name,buildings[i].building_id));
            } 
        }
      }
    xmlhttp.open("GET","/Schedule/json/building/list?campus_id=" + campus_id ,true);
    xmlhttp.send();
}

function getRooms() {
    var building_id = $('#buildingSelect option:selected') .val();
    $("#roomSelect").empty();
    if (typeof(building_id) == "undefined" || building_id == "") {
        alert("请选择教学楼！")
        return;
    }
    var xmlhttp = getXmlHttp();
    xmlhttp.onreadystatechange=function()
      {
      if (xmlhttp.readyState==4 && xmlhttp.status==200)
        {
            var select = document.getElementById("roomSelect");
            var rooms = $.parseJSON(xmlhttp.responseText);
            for(var i = 0; i < rooms.length; i++) {
                select.options.add(new Option(rooms[i].room_name,rooms[i].room_id));
            } 
        }
      }
    xmlhttp.open("GET","/Schedule/json/room/list?building_id=" + building_id ,true);
    xmlhttp.send();
}
//
function getSchedule() {
    var Sel_XNXQ = $('#termSelect option:selected') .val();
    var Sel_XQ = $('#campusSelect option:selected') .val();
    var Sel_JXL = $('#buildingSelect option:selected') .val();
    var Sel_ROOM = $('#roomSelect option:selected') .val()
    if (typeof(Sel_ROOM) == "undefined" || Sel_ROOM == "") {
        alert("请选择需要查询的教室！")
        return;
    }
    var type = $('input:radio[name="type"]:checked').val();
    var txt_yzm = $("#verCode").val();
    $("#verCode").val("");
    $("#roomSchedule").html("");
    $("#picLoading").show();
    $("#picGif").hide();
    var xmlhttp = getXmlHttp();
    xmlhttp.onreadystatechange=function() {
      if (xmlhttp.readyState==4 && xmlhttp.status==200) {
            $("#picLoading").hide();
            var div = document.getElementById("roomSchedule");
            var table = $.parseJSON(xmlhttp.responseText);
            if (table.schedule_content == "null") {
                if ($("#match").is(':hidden')) {
                    $("#match").show();
                }else{
                    alert("请输入验证码！")
                }

            }else if(table.schedule_content == "txt_yzm"){
                refleshValidate(document.getElementById("imgVerCode"));
                alert("验证码错误！");
            }else{
                div.innerHTML = table.schedule_content;
                 $("#match").hide();
                 $("#picGif").hide();
            }

        }
      }
    xmlhttp.open("POST","/Schedule/json/room/course" ,true);
    xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
    xmlhttp.send("Sel_XNXQ=" + Sel_XNXQ + "&Sel_XQ=" + Sel_XQ + "&Sel_JXL=" + Sel_JXL + "&Sel_ROOM=" + Sel_ROOM + "&type=" + type + "&txt_yzm=" + txt_yzm);
}

之前写的下拉列表三级联动的，第一个方法是获取不同浏览器下可以使用的ajax，下面三个方法就是三级联动，先选择校区，根据校区查找教学里，然后选择教学楼，在根据教学楼查教室，最后一个方法是根据教室查找课表信息。