AJAX 中的responseXML为什么一直为空呢？

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AJAX 中的responseXML为什么一直为空呢？

最近在学习AJAX，下面这个例子返回的responseXML总是为空。
下面是构造的一个ajax对象ContentLoader：

1var net=new Object();
2net.READY_STATE_UNINITIALIZED=0;
3net.READY_STATE_LOADING=1;
4net.READY_STATE_LOADED=2;
5net.READY_STATE_INTERACTIVE=3;
6net.READY_STATE_COMPLETE=4;
7net.ContentLoader=function(url,onload,onerror)
8{
9    this.url=url;
10    this.req=null;
11    this.onload=onload;
12    this.onerror=(onerror) ? onerror : this.defaultError;
13    this.loadXMLDoc(url);
14}
15net.ContentLoader.prototype={
16    loadXMLDoc:function(url)
17    {
18        if(window.XMLHttpRequest)
19        { this.req=new XMLHttpRequest();}
20        else if(window.ActiveXObject)
21        { this.req=new ActiveXObject("Microsoft.XMLHTTP");}
22
23      if(this.req)
24      {
25        try{
26        var loader=this;
27        this.req.onreadystatechange=function()
28        {
29            loader.onReadyState.call(loader);
30        }
31        this.req.open("POST",url,true);
32        this.req.send(null);
33        }catch(err){
34        this.onerror.call(this);
35        }
36      }
37    },
38    onReadyState:function()
39    {
40        var req=this.req;
41        var ready=req.readyState;
42        if(ready==net.READY_STATE_COMPLETE)
43        {
44            var httpstatus=req.status;
45            if(httpstatus==200||httpstatus==0)
46            {
47                this.onload.call(this);
48            }
49            else
50            {
51                this.onerror.call(this);
52            }
53        }
54    },
55    defaultError:function()
56    {
57        alert("error fetching data!"
58            +"\n\nreadyState:"+this.req.readState
59            +"\nstatus: "+this.req.status
60            +"\nheaders: "+this.req.getAllResponseHeaders());
61    }
62}

这个是发送请求的页面：

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="DoubleCombo.aspx.cs" Inherits="WebApplication11.DoubleCombo" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>無題のページ</title>

    <script type="text/javascript" src="../ContentLoader.js">
    </script>

    <script type="text/javascript">
    function FillTerritory(oElem,oTarget)
    {
       var strValue=oElem.options[oElem.selectedIndex].value;
       var strParams="?q="+strValue+"&f="+oTarget.form.name+"&e="+oTarget.name;
       var url="DoubleComboXML.aspx"+strParams;
       new net.ContentLoader(url,FillDropDown,null);
    }
    function FillDropDown()
    {
      var xmlDoc = this.req.responseXML.documentElement;
      var xSel = xmlDoc.getElementsByTagName('selectElement')[0];
      var strFName = xSel.childNodes[0].firstChild.nodeValue;
      var strEName = xSel.childNodes[1].firstChild.nodeValue;

      var objDDL = document.forms[strFName].elements[strEName];
      objDDL.options.length=0;

     var xRows = xmlDoc.getElementsByTagName('entry');
     for(var i =0;i<xRows.length;i++)
     {
        var theText = xRows[i].childNodes[0].firstChild.nodeValue;
        var theValue = xRows[i].childNodes[1].firstChild.nodeValue;
        var option = new Option(theText,theValue);

        try
        {
            objDDL.add(option,null);
        }
        catch (e){
            objDDL.add(option,-1);
        }
     }
    }
    </script>

</head>
<body>
    <form id="form1" runat="server">
    <h2>Ajax Demo</h2>
    <select id="ddlRegion" name="ddlRegion" onchange="FillTerritory(this,ddlTerritory)">
        <option value="-1">Pick a Region</option>
        <option value="1">Eastern</option>
        <option value="2">Northern</option>
        <option value="3">Western</option>
        <option value="4">Southern</option>
    </select>
    <select id="ddlTerritory" name="ddlTerritory">
    </select>
    </form>
</body>
</html>

下面这个页面是被请求页面，主要用来返回请求的数据库数据：

  public partial class DoubleComboXML : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            classiccode
        }

        private DataTable FillDataTable(string sql)
        {
            string strConn = "initial Catalog = Northwind;Data Source = 127.0.0.1;Integrated Security = true;";
            SqlDataAdapter cmdl = new SqlDataAdapter(sql, strConn);
            DataSet ds = new DataSet();
            cmdl.Fill(ds);
            cmdl.Dispose();
            return ds.Tables[0];
        }
    }

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原文地址：https://www.cnblogs.com/xixifusigao/p/1214651.html