private static string ConvertToAbsoluteUrls (string html, Uri relativeLocation) {
IHTMLDocument2 doc = new HTMLDocumentClass ();
doc.write (new object [] { html });
doc.close ();

foreach (IHTMLAnchorElement anchor in doc.links) {
IHTMLElement element = (IHTMLElement)anchor;
string href = (string)element.getAttribute ("href", 2);
if (href != null) {
Uri addr = new Uri (relativeLocation, href);
anchor.href = addr.AbsoluteUri;
}
}

foreach (IHTMLImgElement image in doc.images) {
IHTMLElement element = (IHTMLElement)image;
string src = (string)element.getAttribute ("src", 2);
if (src != null) {
Uri addr = new Uri (relativeLocation, src);
image.src = addr.AbsoluteUri;
}
}

string ret = doc.body.innerHTML;

return ret;
}

来源：网络