C# XmlDocument无法加载XHTML字符串，因为出现错误“0”；对未声明实体的引用'；nbsp'&引用；

我使用以下代码将HTTP响应流转换为XmlDocument

HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
HttpWebResponse response = request.GetResponse() as HttpWebResponse;
Stream responseStream = response.GetResponseStream();
StreamReader responseReader = new StreamReader(responseStream);
String responseString = responseReader.ReadToEnd();
Console.WriteLine(responseString);
Int32 htmlTagIndex = responseString.IndexOf("<html",
   StringComparison.OrdinalIgnoreCase);
XmlDocument responseXhtml = new XmlDocument();
responseString = responseString.Substring(htmlTagIndex); // MARK 1
responseString = responseString.Replace("&nbsp", " "); // MARK 2
responseXhtml.LoadXml(responseString);
return responseXhtml;

HttpWebRequest-request=WebRequest.Create（url）为HttpWebRequest；
HttpWebResponse=request.GetResponse（）作为HttpWebResponse；
Stream responseStream=response.GetResponseStream（）；
StreamReader responseReader=新的StreamReader（responseStream）；
字符串responseString=responseReader.ReadToEnd（）；
控制台。写入线（响应线）；
Int32 htmlTagIndex=responseString.IndexOf（“我将直接用于解析html。即使您必须将html转换为xml，也可以使用它

using (WebClient wc = new WebClient())
{
    HtmlAgilityPack.HtmlDocument doc = new HtmlAgilityPack.HtmlDocument();
    doc.LoadHtml(wc.DownloadString("http://www.google.com"));
    doc.OptionOutputAsXml = true;

    StringWriter writer = new StringWriter();
    doc.Save(writer);

    var xDoc = XDocument.Load(new StringReader(writer.ToString()));
}

HTML Agility Pack：谢谢，但是HTML Agility Pack似乎是一个过度杀伤力。任何简单的代码？（X）HTML通常不是XML。
是用HTML定义的实体。你真的需要将其加载为XML吗？嗯，很好，也许我不需要将其转换为XML。