如何从列表框中选择项目将显示在标签中

所以我有一个问题，在哪里把代码在Windows窗体，它会刷新每次我在列表框中选择其他项目?我试图把listBox1，但它崩溃了我的整个程序，我试图把它放在public Form2()，但它显示只是第一个选择项目在label4，不刷新后，我选择其他项目在listBox1。有解决方案吗?

    var selected = listBox1.SelectedItem.ToString();
    label4.Text = selected;

下面的完整代码

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.IO;`
namespace VocaFlash_Japanese
{
    public partial class Form2 : Form
    {
        public List<string> words = new List<string>();
        public string Wor;
        public int State = 0;

        public Form2()
        {
            InitializeComponent();
            loadfile();
            listBox1.DataSource = words;
            //var selected = listBox1.SelectedItem.ToString();
          //  label4.Text = selected;
            hidetext();
        }
        private void hidetext()
        {
            textBox1.Hide();
            textBox2.Hide();
            textBox3.Hide();
            label1.Hide();
            label2.Hide();
            label3.Hide();
        }
        private void showtext()
        {
            textBox1.Show();
            textBox2.Show();
            textBox3.Show();
            label1.Show();
            label2.Show();
            label3.Show();
        }
        public void Save()
        {
            const string sPath = "Data.txt";
            System.IO.StreamWriter sw = new System.IO.StreamWriter(sPath);
            foreach (string item in words)
            {
                sw.WriteLine(item);
            }
            sw.Close();
        }
        private void UpdateListBox(List<string> words)
        {
            //string contents = null;
            List<string> itemAll = new List<string>();
            foreach (string str in words)
            {
                itemAll.Add(str);
            }
        }
        /*public void Save(List<string> words)
        {
            StringBuilder contents = new StringBuilder();
            foreach (string s in words)
            {
                contents.AppendLine(s);
            }
            System.IO.File.WriteAllText(@"Data.txt", contents.ToString());
        }*/
        private void loadfile()
        {
            string line;
            var file = new System.IO.StreamReader("Data.txt");
            while ((line = file.ReadLine()) != null)
            {
                words.Add(line);
            }
        }
       /* private void Remove()
        {
            string contents = null;
            List<string> itemAll = new List<string>();
            foreach (string str in words)
            {
                itemAll.Add(str);
            }
            foreach (string lstitem in listBox1.SelectedItems)
            {
                itemAll.Remove(lstitem);
                //File.AppendAllText(strFile + "Currently In.txt", strName + Environment.NewLine);
            }
            foreach (string s in itemAll)
            { contents += s + Environment.NewLine; }
            System.IO.File.WriteAllText(@"Data.txt", contents);
        }*/
        private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
           var selected = listBox1.SelectedItem.ToString();
            label4.Text = selected;
        }

        private void button4_Click(object sender, EventArgs e)
        {
            Meniu men = new Meniu();
            men.Show();
            this.Hide();
        }
        private void button1_Click(object sender, EventArgs e)
        {
            State += 1;
            switch (State)
            {
                case 1:
                    showtext();
                    break;
                case 2:
                    words.Add(textBox1.Text);
                    listBox1.DataSource = null;
                    listBox1.DataSource = words;
                    Save();
                    State = 0;
                    textBox1.Text = "";
                    hidetext();
                    break;
                default:
                    Console.WriteLine("Default case");
                    break;
            }
        }
        private void button2_Click(object sender, EventArgs e)
        {
           /* // The Remove button was clicked.
            int selectedIndex = listBox1.SelectedIndex;
            try
            {
                // Remove the item in the List.
                words.RemoveAt(selectedIndex);
            }
            catch
            {
            }
            listBox1.DataSource = null;
            listBox1.DataSource = words;
            Remove();*/
            // The Remove button was clicked.
            int selectedIndex = listBox1.SelectedIndex;
            try
            {
                // Remove the item in the List.
                words.RemoveAt(selectedIndex); //Remove item from words
                UpdateListBox(words);//Update contents on GUI
                Save(); //Save on IO
                listBox1.DataSource = null;
                listBox1.DataSource = words;
            }
            catch
            {
            }

        }
        private void Form2_FormClosing(object sender, FormClosingEventArgs e)
        {
            Environment.Exit(0);
        }
        private void button3_Click(object sender, EventArgs e)
        {
        }
        private void label4_Click(object sender, EventArgs e)
        {
        }
    }
}