C#中xml的取值


try
            {
                Encoding myEncoding = Encoding.GetEncoding("UTF-8");
                string param = "action=query&userid=" + HttpUtility.UrlDecode(userid.Text, myEncoding) + "&account=" + HttpUtility.UrlDecode(account.Text, myEncoding) + "&password=" + HttpUtility.UrlDecode(password.Text, myEncoding);
                byte[] postBytes = Encoding.ASCII.GetBytes(param);
                HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create("http://sdk.kuai-xin.com:8888/sms.aspx");
                req.Method = "POST";
                req.ContentType = "application/x-www-form-urlencoded;charset=UTF-8";
                req.ContentLength = postBytes.Length;
                using (Stream reqStream = req.GetRequestStream())
                {
                    reqStream.Write(postBytes, 0, postBytes.Length);
                }
                System.Xml.XmlDocument xmlDoc = new System.Xml.XmlDocument();
                using (WebResponse wr = req.GetResponse())
                {
                    StreamReader sr = new StreamReader(wr.GetResponseStream(), System.Text.Encoding.UTF8);
                    System.IO.StreamReader xmlStreamReader = sr;
                    xmlDoc.Load(xmlStreamReader);
                }
                if (xmlDoc == null)
                {
                    MessageBox.Show("请求发生异常");
                }
                else
                {
                  
       
                }
            }
            catch (System.Net.WebException WebExcp)
            {
                MessageBox.Show("网络错误，无法连接到服务器！");
            }
        }xml的返回值为<?xml version="1.0" encoding="utf-8" ?> 
<returnsms>
<callbox>
<mobile>15023239810</mobile>-------------对应的手机号码
<taskid>1212</taskid>-------------同一批任务ID
<content>你好，我不需要</content>---------上行内容
<receivetime>2011-12-02 22:12:11</receivetime>-------------接收时间
</callbox>
<callbox>
<mobile>15023239811</mobile>
<taskid>1212</taskid>
<content>你好，本次活动路线是怎么的</content>
<receivetime>2011-12-02 22:12:11</receivetime>
</callbox>
</returnsms>谁给我看下如何提取出来mobile，taskid这里面的值呢？C#XML