I'm designing a web crawler application, I need some help regarding this app

See more:

C#

I'm desiging this app,

private string fetch(string add, string url)
        {
            string fet = string.Empty;
            if (add.Length > 0)
            {
                HtmlWeb htmlweb = new HtmlWeb();
                List<string> a = new List<string>();
               
                HtmlAgilityPack.HtmlDocument doc = htmlweb.Load(add);
                if (doc != null && doc.DocumentNode != null)
                {
                    HtmlNodeCollection link = doc.DocumentNode.SelectNodes("//a[@href]");
                    if (link != null)
                    {
                        foreach (HtmlNode li in link)
                        {
                            HtmlAttribute att = li.Attributes["href"];
                            if (att.Value.StartsWith(url) == true)
                            {
                                fet += att.Value + ",";
                            }
                            else if (att.Value.StartsWith("/") == true)
                            {
                                fet += url + att.Value + ",";
                            }
                            else if (att.Value.Contains("#comment") == true || att.Value.Contains("#respond") == true || att.Value.Contains("/refer/"))
                            {

                            }
                        }
                    }
                }
            }
            else
            {
                return "";
            }
            return fet;
        }

private void button1_Click(object sender, EventArgs e)
        {
            HtmlWeb hw = new HtmlWeb();
            List<string> a = new List<string>();
            List<string> link2 = new List<string>();
            HtmlAgilityPack.HtmlDocument doc = hw.Load(textBox1.Text);

            foreach (HtmlNode link in doc.DocumentNode.SelectNodes("//a[@href]"))
            {
                HtmlAttribute att = link.Attributes["href"];
                
                a.Add(att.Value);
            }
            //using (StreamWriter writer = new StreamWriter(@"C:\Users\M Adeel\Documents\Visual Studio 2010\Projects\CrawlingLinks\Projects\test.txt"))
            //{
                foreach (string s in a)
                {
                    if (string.IsNullOrWhiteSpace(s) == true)
                    {

                    }
                    else
                    {
                        if (s.StartsWith(textBox1.Text) == true)
                        {
                            link2.Add(s);
                        }
                        else if (s.StartsWith("/") == true)
                        {
                            link2.Add(textBox1.Text + s);
                        }
                    }

                }
                string ret = string.Empty, ret2= string.Empty;
                foreach (string s in link2)
                {
                    ret+=fetch(s, textBox1.Text);
                }
                string[] ar = ret.Split(',');
                string[] q1 = ar.Distinct().ToArray();
                //foreach (string s in q1)
                //{
                //    ret2 += fetch(s, textBox1.Text);
                //}
                //string[] q2 = ar.Distinct().ToArray();
                
            using (StreamWriter writer = new StreamWriter(@"C:\Users\M Adeel\Documents\Visual Studio 2010\Projects\CrawlingLinks\Projects\test.txt"))
                {
                    foreach (string s in q1)
                    {
                        writer.Write(s + "\r\n");
                    }
                }
        }

Let me explain this code first, I'm using htmlagility pack for extracting URLs from a website. It almost works but not fully. The button code first fetches the data of a given url within a text box using htmlagilitypack and saves the links in a list, it then further manipulates the links. In a foreach loop i call the function fetch with 2 parameters namely add and url, add is the inner link of some site like http://www.example.com/page1/ etc and url is http://www.example.com which is entered by the user. The problem here is that, it fetches the links but i need the code to be modified so, it can recursively call inner pages of the website and so, crawl it until the last inner url of website, How can i modify it? my logic is poor regarding this situation. I gave it a domain name like http://www.example.com it goes to some page like http://www.example.com/blog/page1 and /blog/page2 and directly goes to the last page /blog/page144 without fetching one by one, actually 3 pages are mentionaed at the blog page of that website, so i think, recursive call should be best rather than making inefficient and time consuming code. I hope you'll understand my query.
It doesn't crawl a website as a whole. I've been looking around for some solution but unable to find any.
Regards!