private string nev;
        private List<string> iranyok = new List<string>();
        private List<List<string>> jaratok = new List<List<string>>();

        public string Nev {
            get { return nev; }
            set { nev = value; }
        }
        public List<string> Iranyok {
            get { return iranyok; }
            set { iranyok = value; }
        }
        public List<List<string>> Jaratok {
            get { return jaratok; }
            set { jaratok = value; }
        }
----------
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;

namespace a7_busz
{
    public partial class _default : System.Web.UI.Page
    {
        private List<jarat> buszok = new List<jarat>();

        protected void Page_Load(object sender, EventArgs e)
        {
            string[] files = Directory.GetFiles(Server.MapPath("~/jaratok/"), "*.txt");

            foreach(string file in files) {
                if (!IsPostBack)
                    DropDownList1.Items.Add(Path.GetFileNameWithoutExtension(file));
                jarat j = new jarat();
                j.Nev = Path.GetFileNameWithoutExtension(file);
                using (StreamReader sr = new StreamReader(file)) {
                    string line;
                    while ((line = sr.ReadLine()) != null) {
                        if (line.Substring(0,1) == "I")
                            j.Iranyok.Add(line.Substring(7));
                        else {
                            if (j.Jaratok.Count < j.Iranyok.Count)
                                j.Jaratok.Add(new List<string>());
                            j.Jaratok[j.Jaratok.Count - 1].Add(line);
                        }
                    }
                }
                buszok.Add(j);
            }
             if (!IsPostBack) resetDrop2();
        }

        private void resetDrop2() {
            List<string> iranyok = new List<string>();
            foreach (jarat j in buszok)
                if (j.Nev == DropDownList1.SelectedValue)
                    iranyok = j.Iranyok;
            DropDownList2.DataSource = iranyok;
            DropDownList2.DataBind();
        }

        protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e) {
            resetDrop2();
        }
        protected void Button1_Click(object sender, EventArgs e) {
            foreach (jarat j in buszok)
                if (j.Nev == DropDownList1.SelectedValue)
                    for (int i = 0; i < j.Iranyok.Count; i++)
                        if (j.Iranyok[i] == DropDownList2.SelectedValue) {
                            GridView1.DataSource = j.Jaratok[i].Select(item => new { jaratok = item });
                            GridView1.DataBind();
                        }
        }
    }
}