string yourHtml = string.Empty;
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(@"https://www.google.com.vn/?gfe_rd=cr&ei=jDp1VOSdBa6tmQWK5YA4&gws_rd=ssl");
    using (Stream stream = request.GetResponse().GetResponseStream())
    using (StreamReader streamReader = new StreamReader(stream))
    {
        yourHtml = streamReader.ReadToEnd();
    }