string InfoUrl = "http://cn.bing.com/HPImageArchive.aspx?idx=0&n=1";
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(InfoUrl);
request.Method = "GET"; request.ContentType = "text/html;charset=UTF-8";
string XmlString;
using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
{
  Stream myResponseStream = response.GetResponseStream();
  using (StreamReader myStreamReader = new StreamReader(myResponseStream, Encoding.UTF8))
  {
    XmlString = myStreamReader.ReadToEnd();
  }
}
// 定义正则表达式用来匹配标签
Regex regImg = new Regex("<Url>(?<imgUrl>.*?)</Url>", RegexOptions.IgnoreCase);
// 搜索匹配的字符串
MatchCollection matches = regImg.Matches(XmlString);
// 取得匹配项列表
string ImageUrl = "http://www.bing.com" + matches[0].Groups["imgUrl"].Value;
background_image.Src = ImageUrl;