ColorPicker/src/ScreenColorPickerForm.cs

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.Windows.Forms;

namespace ColorPickerApp
{
    /// <summary>
    /// Superpose une fenetre transparente plein ecran (tous moniteurs) permettant
    /// de capturer la couleur de n'importe quel pixel affiche a l'ecran.
    /// </summary>
    internal sealed class ScreenColorPickerForm : Form
    {
        private readonly Timer _timer;
        private readonly MagnifierForm _magnifier;
        private Color _hoverColor = Color.White;
        private Point _lastScreenPoint;

        public Color PickedColor { get; private set; }

        public ScreenColorPickerForm()
        {
            FormBorderStyle = FormBorderStyle.None;
            ShowInTaskbar = false;
            StartPosition = FormStartPosition.Manual;
            TopMost = true;
            BackColor = Color.Black;
            Opacity = 0.01; // fenetre quasi invisible, sert uniquement a capter la souris
            Cursor = Cursors.Cross;
            DoubleBuffered = true;
            KeyPreview = true;

            Bounds = SystemInformation.VirtualScreen;

            _magnifier = new MagnifierForm();

            _timer = new Timer { Interval = 40 };
            _timer.Tick += (s, e) => UpdateHover();

            MouseClick += OnMouseClick;
            KeyDown += (s, e) => { if (e.KeyCode == Keys.Escape) { PickedColor = Color.Empty; Close(); } };
        }

        protected override void OnShown(EventArgs e)
        {
            base.OnShown(e);
            _magnifier.Show(this);
            _timer.Start();
        }

        protected override void OnFormClosed(FormClosedEventArgs e)
        {
            _timer.Stop();
            _magnifier.Close();
            base.OnFormClosed(e);
        }

        private void UpdateHover()
        {
            _lastScreenPoint = Cursor.Position;
            try
            {
                using (var bmp = new Bitmap(1, 1, PixelFormat.Format32bppArgb))
                {
                    using (var g = Graphics.FromImage(bmp))
                    {
                        g.CopyFromScreen(_lastScreenPoint, Point.Empty, new Size(1, 1));
                    }
                    _hoverColor = bmp.GetPixel(0, 0);
                }
            }
            catch
            {
                // ignore capture errors (ex: DPI change) et garder la derniere couleur connue
            }

            _magnifier.UpdateDisplay(_lastScreenPoint, _hoverColor);
        }

        private void OnMouseClick(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                PickedColor = _hoverColor;
                Close();
            }
            else
            {
                PickedColor = Color.Empty;
                Close();
            }
        }
    }
}