ajax读取xml的问题

后天controllers中用xmlDocument 创建一个xml文件， 在页面用xmlhttp.responseXml得到的数据 alert（）提示为undefined
生成xml的代码
   public XmlDocument ajaxReadXML()
        {
            XmlDocument xd = this.getXMLDocument();
            return xd;
        }

页面ajax代码
     var xmlhttp = null;
        function testAjaxReadXML() {
            if (window.XMLHttpRequest) {
                xmlhttp = new XMLHttpRequest();
            } else if (window.ActiveXObject) {
            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
            } else {
                alert("浏览器不支持！");
            }
            if (xmlhttp != null) {
                xmlhttp.onreadystatechange = state_Change;
                xmlhttp.open("get", "ajaxReadXML", true);
                xmlhttp.send(""); 
            }

        } 
        function state_Change() {
                     if (xmlhttp.readyState == 4) {
                if (xmlhttp.status == 200) {
                    var xmlDoc = xmlhttp.responseXml
                    alert(xmlDoc);
            //        document.getElementById("content").innerHTML = xmlDoc.getElementsByTagName("title")[0].childNodes[0].nodeValue;
                } else {
                alert("XML 数据错误:" + xmlhttp.statusText);
                }
            }

补充：.NET技术 ,  .NET Framework