【.Net5】WinFormで時計

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

namespace Clock3
{
    public partial class Form1 : Form
    {
        PictureBox picbox1 = new PictureBox
        {
            Dock = DockStyle.Fill,
        };
        Button button1 = new Button
        {
            Dock = DockStyle.Bottom,
            Text = "Start",
            Height = 80,
        };
        Bitmap bmp = null;
        Font fnt = new Font("MS UI Gothic", 48);

        System.Threading.Timer timer;

        void timerCallBack(Object state)
        {
            if (picbox1 == null) return;
            if (picbox1.InvokeRequired)
            {
                Invoke(new Action(() =>
                {
                    lock(this)
                    {
                        if (this.bmp == null)
                        {
                            this.bmp = new Bitmap(this.picbox1.Width, this.picbox1.Height);
                        }
                        using(var g = Graphics.FromImage(this.bmp))
                        {
                            DateTime dt = DateTime.Now;
                            string text = dt.ToString("HH:mm:ss");
                            SizeF stringSize = g.MeasureString(text, fnt);
                            int x = (bmp.Width - stringSize.ToSize().Width) / 2;
                            int y = (bmp.Height - stringSize.ToSize().Height) / 2;
                            g.FillRectangle(
                                Brushes.White, x, y, stringSize.ToSize().Width, stringSize.ToSize().Height);
                            g.DrawString(text, fnt, Brushes.Black, x, y);
                        }
                        this.picbox1.Invalidate();

                    }
                }));
            }
        }
        public Form1()
        {
            InitializeComponent();

            this.Controls.Add(button1);
            this.Controls.Add(picbox1);

            this.timer = new System.Threading.Timer(
                timerCallBack, null, Timeout.Infinite, Timeout.Infinite);

            this.picbox1.Resize += (s, e) =>
            {
                lock(this)
                {
                    if (this.bmp != null)
                    {
                        this.bmp.Dispose();
                        this.bmp = null;
                    }
                }
            };
            this.picbox1.Paint += (s, e) =>
            {
                lock(this)
                {
                    if (this.bmp != null)
                    {
                        e.Graphics.DrawImageUnscaled(this.bmp, 0, 0);
                    }
                }
            };
            this.FormClosing += (s, e) =>
            {
                if (this.timer != null)
                {
                    this.timer.Change(Timeout.Infinite, Timeout.Infinite);
                    this.timer.Dispose();
                }
            };
            this.button1.Click += (s, e) =>
            {
                if (this.button1.Text == "Start")
                {
                    this.button1.Text = "Stop";
                    this.timer.Change(0, 1000);
                }
                else
                {
                    this.button1.Text = "Start";
                    this.timer.Change(Timeout.Infinite, Timeout.Infinite);
                }
            };
        }

    }
}