OMControlCs/OMControl/Form1.cs

using System.IO.Ports;

namespace OMControl
{
    public partial class Form1 : Form
    {
        private SerialPortWatcher _watcher;
        public Form1()
        {
            InitializeComponent();
            this.Load += Form1_Load;

        }
        

        private void Form1_Load(object sender, EventArgs e)
        {
            listBoxPorts.Items.Clear();
            foreach (var p in SerialPort.GetPortNames())
                listBoxPorts.Items.Add(p);

            // start watcher
            _watcher = new SerialPortWatcher(1000);

            _watcher.PortsAdded += ports => BeginInvoke(() =>
            {
                foreach (var p in ports)
                    if (!listBoxPorts.Items.Contains(p))
                        listBoxPorts.Items.Add(p);
            });

            _watcher.PortsRemoved += ports => BeginInvoke(() =>
            {
                foreach (var p in ports)
                    listBoxPorts.Items.Remove(p);
            });

            _watcher.Start();
        }
        protected override void OnFormClosing(FormClosingEventArgs e)
        {
            _watcher?.Dispose();
            base.OnFormClosing(e);
        }
    }
}