To Do w/ Listbox

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

namespace Test
{
    public partial class Generator : Form
    {
        public Generator()
        {
            InitializeComponent();
            this.AutoSizeMode = AutoSizeMode.GrowAndShrink;
            this.AutoSize = true;
            this.Padding = new Padding(0, 0, 20, 30);
            this.StartPosition = FormStartPosition.CenterScreen;




        }
        private ArrayList arrNewToDO = new ArrayList();
        private Int32 i = 0;
        Label labelInput = new Label();



        private void btnGen_Click(object sender, EventArgs e)
        {
            String toDo = txtNumber.Text;
            lBToDo.Items.Add(toDo);




            i++;





        }

        private void lBToDo_SelectedIndexChanged(object sender, EventArgs e)
        {
            lBToDo.Items[lBToDo.SelectedIndex -1 +1] = "Hi";



        }

    }
}