求助，存取html页面的问题，简单的说。

读取 html到DOM ,找到tag中span中的id属性，去更改id的属性
debugview中看到更改，但是存为文件，没有更改

该如何写呢？谢谢，代码如下
public void ParserHtmlResource(string path)
{

System.IO.StreamReader sr = new System.IO.StreamReader(path);
string strHtml = sr.ReadToEnd();
mshtml.IHTMLDocument2 mydoc = new mshtml.HTMLDocumentClass();

mydoc.write(strHtml);

foreach (mshtml.IHTMLElement el in (mshtml.IHTMLElementCollection)mydoc.body.all)
{
string tagname = el.tagName;
if (tagname.ToLower() == "span")
{
object nameAttribute = el.getAttribute("id", 0);
if (nameAttribute != null)
{
// mshtml.IHTMLElement3 element = (mshtml.IHTMLElement3)el;
// element.contentEditable = "true";
Random rand = new Random();
int serial = rand.Next(500);
string name = string.Format("unnamedid{0:D}", serial);
el.setAttribute("id", name, 1);
}
}
}
IPersistFile ss = mydoc as IPersistFile;
ss.Save("c:\\cc.html", true);
}