App.xaml.cs

using System;
using System.Drawing;
using System.Windows;
using Forms = System.Windows.Forms;

namespace MinimalSittingNotifier {
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application {
        private const int MaxSitMinute = 30;
        private       int _timeCountNumber;

        private readonly Font  _defaultIconFont      = new Font("Calibri", 8);
        private readonly Color _defaultIconFontColor = Color.White;
        private readonly Color _defaultIconStopColor = Color.Yellow;
        private const    int   IconSize              = 16;

        private System.Timers.Timer _timer;

        // private int h, m, s;
        private readonly Forms.NotifyIcon _notifyIcon;

        public App() {
            _notifyIcon = new Forms.NotifyIcon();
        }

        protected override void OnStartup(StartupEventArgs e) {
            ShowNumberIcon(MaxSitMinute.ToString());
            _notifyIcon.ContextMenuStrip = new Forms.ContextMenuStrip();
            _notifyIcon.ContextMenuStrip.Items.Add("Exit", null, ExitProgram);
            _notifyIcon.MouseClick += ReStartTimer;
            _notifyIcon.Visible    =  true;
            StartTimer();

            base.OnStartup(e);
        }

        private void StartTimer() {
            _timeCountNumber = MaxSitMinute;

            _timer         =  new System.Timers.Timer(1000 * 60); //1 sec * 60 = 1 min interval;
            _timer.Elapsed += OnTimedEvent;
            _timer.Enabled =  true;
            ShowNumberIcon(_timeCountNumber.ToString());
        }

        private void ReStartTimer(object? sender, Forms.MouseEventArgs e) {
            if (e.Button == Forms.MouseButtons.Left) {
                _timer.Dispose();
                StartTimer();
            }
        }

        private void OnTimedEvent(object? sender, System.Timers.ElapsedEventArgs e) {
            if (_timeCountNumber <= 1) {
                _timer.Enabled = false;
                ShowStopIcon();
            }
            else {
                _timeCountNumber--;
                ShowNumberIcon(_timeCountNumber.ToString());
            }
        }

        private void ExitProgram(object? sender, EventArgs e) {
            _notifyIcon.Visible = false;
            _notifyIcon.Dispose();
            Environment.Exit(0);
        }

        private void ShowNumberIcon(String numberText) {
            Brush    brush    = new SolidBrush(_defaultIconFontColor);
            Bitmap   bitmap   = new Bitmap(IconSize, IconSize);
            Graphics graphics = Graphics.FromImage(bitmap);
            graphics.DrawString(numberText, _defaultIconFont, brush, 0, 0);

            Icon icon = Icon.FromHandle(bitmap.GetHicon());
            _notifyIcon.Icon = icon;
        }

        private void ShowStopIcon() {
            Brush    brush    = new SolidBrush(_defaultIconStopColor);
            Bitmap   bitmap   = new Bitmap(IconSize, IconSize);
            Graphics graphics = Graphics.FromImage(bitmap);
            graphics.FillRectangle(brush, 0, 0, IconSize, IconSize);

            var icon = Icon.FromHandle(bitmap.GetHicon());
            _notifyIcon.Icon = icon;
        }

        protected override void OnExit(ExitEventArgs e) {
            _notifyIcon.Visible = false;
            _notifyIcon.Dispose();
            base.OnExit(e);
        }
    }
}