如何把html中的相对路径变成绝对路径

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;
}