How do I recover the full html of a page, including what is generated by javascript

How do I recover the full html of a page, including what is generated by
javascript

How do I recover the full html of a page, including what is generated by
javascript. The problem is that I want to access the contents of the
select tag, but the page but it is coming empty, this probably being
generated dynamically. Please I'm about to give up! I just posted a piece
of code because this very large, if I find it necessary to put the whole
code.
res = (HttpWebResponse)req.GetResponse();
res.Cookies = req.CookieContainer.GetCookies(req.RequestUri);
cookieContainer.Add(res.Cookies);
sr = new StreamReader(res.GetResponseStream());
getHtml = sr.ReadToEnd();
viewstate = rxViewstate.Match(getHtml).Groups[1].Value;
EventValdidation = rxEventValidation.Match(getHtml).Groups[1].Value;
viewstate = HttpUtility.UrlEncode(viewstate);
EventValdidation = HttpUtility.UrlEncode(EventValdidation);
//Here I should take the contents of the select tag.
getHtml = rxDropDownMenu.Match(getHtml).Groups[2].Value;