OOP LV6 - Zadatak2

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace LV6___Zadatak_2
{
    public partial class form_imenik : Form
    {
        public form_imenik()
        {
            InitializeComponent();
        }

        class Kontakt
        {
            #region data_members
            private string ime;
            public string prezime;
            private string broj_telefona;
            #endregion

            #region public_methods
            public Kontakt()
            {
                ime = ""; prezime = ""; broj_telefona = "";
            }
            public Kontakt(string i, string p, string b)
            {
                ime = i; prezime = p; broj_telefona = b;
            }
            public override string ToString()
            {
                return ime + "\t" + prezime + "\t" + broj_telefona;
            }
            #endregion
        }

        List<Kontakt> listKontakt = new List<Kontakt>();
        string path = "C:\\Users\\Student\\Documents\\Visual Studio 2015\\Projects\\LV6 - Zadatak 2\\LV6 - Zadatak 2\\imenik.txt";

        private void form_imenik_Load(object sender, EventArgs e)
        {
            using (System.IO.StreamReader reader = new System.IO.StreamReader(@path))
            {
                string line;
                while ((line = reader.ReadLine()) != null)
                {
                    string[] parts = line.Split('\t');
                    Kontakt K = new Kontakt(parts[0], parts[1], parts[2]);
                    listKontakt.Add(K);
                }
                listBox_popis.DataSource = null;
                listBox_popis.DataSource = listKontakt;
            }

        }

        private void button_dodaj_Click(object sender, EventArgs e)
        {
            bool error = false;

            if (textBox_ime.Text == "" || textBox_prezime.Text == "" || textBox_broj.Text == "")
            {
                error = true;
                MessageBox.Show("Neko od polja ostalo je prazno.", "Pogreška!");
            }
            else if (!Regex.IsMatch(textBox_broj.Text, @"^[0-9]+$"))
            {
                error = true;
                MessageBox.Show("Broj telefona je neispravan!", "Pogreška!");
            }
            else
            {
                error = false;
            }

            if (!error)
            {
                Kontakt K = new Kontakt(textBox_ime.Text, textBox_prezime.Text, textBox_broj.Text);
                listKontakt.Add(K);
                listBox_popis.DataSource = null;
                listBox_popis.DataSource = listKontakt;
                textBox_ime.Text = String.Empty;
                textBox_prezime.Text = String.Empty;
                textBox_broj.Text = String.Empty;
            }
        }

        private void button_izlaz_Click(object sender, EventArgs e)
        {
            using (System.IO.StreamWriter write = new System.IO.StreamWriter(@path))
            {
                foreach (Kontakt K in listKontakt)
                {
                    {
                        write.WriteLine(K.ToString());
                    }
                }
            }
            Application.Exit();
        }

        private void button_pretraga_Click(object sender, EventArgs e)
        {
            bool state = false;

            if (textBox_pretraga.Text == "")
            {
                MessageBox.Show("Niste unjeli prezime!", "Pogreška!");
            }
            else
            {
                foreach (Kontakt K in listKontakt)
                {
                    if (K.prezime.Contains(textBox_pretraga.Text))
                    {
                        state = true;
                        break;
                    }
                    else
                    {
                        state = false;
                    }
                }

                if (state)
                {
                    MessageBox.Show("Kontakt postoji!", "Pretraga!");
                }
                else
                {
                    MessageBox.Show("Kontakt ne postoji!", "Pretraga!");
                }
            }
        }
    }
}