如上面的MatthewG所述，答案已经发布到

As noted by MatthewG above, the answer had already been posted to html entity encode text only, not html tag - the solution is to use HTMLAgilityPack and pass the html text or node to the method entitize - this encodes only the page content and not the tags.

using HtmlAgilityPack;
html = HtmlEntity.Entitize(html);