SimpleWebCrawler with 2 node deep - console app

Simple.WebCrawler.cs

class Program
{
    static void Main(string[] args)
    {
        LinkCrawler crawler = new LinkCrawler();
        List<string> list = crawler.Catch("http://blog.yemrekeskin.com/en/");

        foreach (var item in list)
        {
            Console.WriteLine(item);
            List<string> nestedList = crawler.Catch(item);
            foreach (var nestedItem in nestedList)
            {
                Console.WriteLine("----" +nestedItem);    
            }
        }
            
        Console.ReadLine();
    }
}

public interface ILinkCrawler
{
    List<string> Catch(string link);
}

public class LinkCrawler
    : ILinkCrawler
{
    public List<string> Catch(string link)
    {
        if (String.IsNullOrEmpty(link))
            throw new ApplicationException("");
            
        List<string> rl = new List<string>();

        if (link.IsImage())
            return rl;

        StringHelper.CheckLinkControl(ref link);

        WebRequest wr = WebRequest.Create(link);
        StreamReader sr = null;
        WebResponse ws = null;
        string response = String.Empty;

        try
        {
            ws = wr.GetResponse();
            sr = new StreamReader(ws.GetResponseStream(), Encoding.UTF8);
            response = sr.ReadToEnd();
        }
        catch (Exception ex)
        {
            //throw new ApplicationException(ex.Message);
        }
        finally
        {
            if (sr != null) sr.Close();
            if (ws != null) ws.Close();
        }

        Regex r = new Regex("<a.+href=\"http.+://(.+)\">(.*)</a>");
        MatchCollection mc = r.Matches(response);
        string dummyLink = string.Empty;
        foreach (Match m in mc)
        {
            dummyLink = m.Groups[1].Value;

            if (dummyLink.IndexOf("\"") > -1)
                dummyLink = dummyLink.Substring(0, dummyLink.IndexOf("\""));

            rl.Add(dummyLink);
            dummyLink = string.Empty;
        }

        return rl;
    }
}

public static class ImageExtention
{
    public static readonly List<string> ImageExtensions = new List<string> { ".JPG", ".JPE", ".BMP", ".GIF", ".PNG" };

    public static bool IsImage(this string url) 
    {
        return ImageExtensions.Contains(Path.GetExtension(url).ToUpperInvariant());                        
    }
}

public static class StringHelper
{ 
    public static void CheckLinkControl(ref string link)
    {
        string web = "http://";
        if (link.Substring(0, 7) != web)
            link = link.Insert(0, web);
    }
}