public class LinksHolder
{
    private const int ErrorCode = -1;
    private const int noErrorCode = 0;
    private const int levelOutCode = 1;
    public string Title { get; set; }
    public string Url { get; set; }
    public List<LinksHolder> ListOfChildLinks { get; set; }

    public LinksHolder()
    {
    }
    public int Create(LinkItem _linkItem)
    {
        //if (level <= 0) return levelOutCode;
        if (_linkItem.ErrorString == "error: no_title" || _linkItem.Title == ""
            || _linkItem.ErrorString == "error: invalid_url" || _linkItem.Url == "")
        {
            return ErrorCode;
        }
        ListOfChildLinks = new List<LinksHolder>();
        this.Title = _linkItem.Title;
        this.Url = _linkItem.Url;
        foreach (string link in _linkItem.ListOfUrl)
        {
            LinksHolder tempLinksHolderInstance = new LinksHolder();
            //int code = tempLinksHolderInstance.Create(HtmlParser.Parse(link), level - 1);
            //if (code == noErrorCode)
            //    ListOfChildLinks.Add(tempLinksHolderInstance);
            //else if (code == levelOutCode)
            //{
            //    return levelOutCode;
            //}
            tempLinksHolderInstance = Add1(link);
            if (tempLinksHolderInstance != null)
                ListOfChildLinks.Add(this.Add1(link));

        }
        return noErrorCode;
    }

    public LinksHolder Add1(string _url)
    {
        LinksHolder tempLinksHolderInstance = new LinksHolder();
        LinkItem linkItemInstance = HtmlParser.Parse(_url);
        if (linkItemInstance.ErrorString == "error: no_title" || linkItemInstance.Title == ""
            || linkItemInstance.ErrorString == "error: invalid_url" || linkItemInstance.Url == "")
        {
            return null;
        }
        tempLinksHolderInstance.Title = linkItemInstance.Title;
        tempLinksHolderInstance.Url = linkItemInstance.Url;
        return tempLinksHolderInstance;
    }
}


public struct LinkItem
{
    public string Title;
    public string Url;
    public string ErrorString;
    public List<string> ListOfUrl;
}
static public class HtmlParser
{
    private const string _urlRegex = @"\b(?:https?://|www\.)\S+\b|href=\""(.*?)\""";
    private const string _titleRegex = @"\<title\b[^>]*\>\s*(?<Title>[\s\S]*?)\</title\>";
    static public LinkItem Parse(string _url)
    {
        if (HtmlParser.СheckUrl(_url) == false)
            return new LinkItem { ErrorString = "error: invalid_url", Url = _url };
        HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(_url);
        string htmlPageString;
        WebClient client = new WebClient();
        client.Encoding = System.Text.Encoding.UTF8;
        try
        {
            htmlPageString = client.DownloadString(_url);
        }
        catch
        {
            return new LinkItem { ErrorString = "error: invalid_url", Url = _url };
        }
        MatchCollection links = Regex.Matches(htmlPageString, _urlRegex, RegexOptions.Singleline);
        LinkItem linkItemInstance = new LinkItem();
        linkItemInstance.ListOfUrl = new List<string>();
        string title = Regex.Match(htmlPageString, _titleRegex, RegexOptions.IgnoreCase).Groups["Title"].Value;
        if (title == "")
        {
            return new LinkItem { ErrorString = "error: no_title", Url = _url };
        }
        linkItemInstance.Url = (_url);
        linkItemInstance.Title = title;

        foreach (Match item in links)
        {
            string urlString = item.Groups[1].Value;
            if (urlString != "" && urlString != @"/" && urlString != request.Address.AbsolutePath
                && urlString != request.Address.AbsoluteUri && urlString != @"//" && urlString[0] != '#')
            {
                if (HtmlParser.СheckUrl(urlString) == false)
                {
                    if (urlString[0] != '/')
                    {
                        urlString = request.Address.Scheme + @"://" + request.Address.Host + "/" + urlString;
                    }
                    else
                    {
                        urlString = request.Address.Scheme + @"://" + request.Address.Host + urlString;
                    }
                }
                if (HtmlParser.СheckUrl(urlString) == true)
                {
                    linkItemInstance.ListOfUrl.Add(urlString);
                }
            }
        }
        return linkItemInstance;
    }

    public static bool СheckUrl(string UrlString)
    {
        Uri uriResult;
        bool result = Uri.TryCreate(UrlString, UriKind.Absolute, out uriResult)
            && (uriResult.Scheme == Uri.UriSchemeHttp || uriResult.Scheme == Uri.UriSchemeHttps);
        return result;
    }
}