VisualComponents/VisualComponentsLib/MyListBox.cs

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;

namespace VisualComponentsLib.CustomListBox
{
    [DefaultEvent(nameof(TextChanged))]
    public partial class MyListBox : UserControl
    {
        [Browsable(true)]
        public string SelectedString
        {
            get
            {
                if (listBox.SelectedItems.Count == 0)
                {
                    return null;
                }
                else
                {
                    return listBox.SelectedItem.ToString();
                }
            }

            set
            {
                if (!string.IsNullOrEmpty(value) && listBox.Items.IndexOf(value) != -1)
                {
                    listBox.SelectedItem = value;
                }
            }
        }

        public MyListBox()
        {
            InitializeComponent();
        }

        public void AddItem(string item)
        {
            if (!string.IsNullOrEmpty(item))
            {
                listBox.Items.Add(item);
            }
        }

        public void ClearAll()
        {
            listBox.Items.Clear();
        }

        private EventHandler _textChanged;

        public event EventHandler? TextChanged
        {
            add => _textChanged += value;
            remove => _textChanged -= value;
        }

        private void ListBox_SelectedIndexChanged(object sender, EventArgs e)
        {
            _textChanged?.Invoke(sender, e);
        }
    }
}