FlatCheckBox.cs

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;

namespace GigaPrime3D
{
    /// <summary>
    /// Owner-drawn dark check box used in place of the stock WinForms CheckBox so
    /// the sidebar matches the WPF demo's dark scientific look.
    ///
    /// Replaces the WPF CheckBox whose Checked / Unchecked event pair becomes a
    /// single CheckedChanged event here (reading .Checked). Setting Checked to its
    /// current value is a no-op, so the _updatingUi guard pattern works.
    /// </summary>
    public class FlatCheckBox : Control
    {
        // ---- Theme -------------------------------------------------------------
        public static Color BoxColor     = Color.FromArgb(255, 0x3C, 0x42, 0x4A); // box fill (unchecked)
        public static Color BoxBorder    = Color.FromArgb(255, 0x6A, 0x72, 0x7C);
        public static Color AccentColor  = Color.FromArgb(255, 0x5A, 0xAA, 0xEB); // box fill (checked) + check
        public static Color CheckColor   = Color.FromArgb(255, 0xFF, 0xFF, 0xFF);
        public static Color LabelColor   = Color.FromArgb(255, 0xFF, 0xFF, 0xFF);
        // ------------------------------------------------------------------------

        private bool _checked = false;
        private const int BoxSize = 18;
        private const int Gap = 8;

        public event EventHandler CheckedChanged;

        public FlatCheckBox()
        {
            SetStyle(ControlStyles.AllPaintingInWmPaint | ControlStyles.UserPaint |
                     ControlStyles.OptimizedDoubleBuffer | ControlStyles.ResizeRedraw |
                     ControlStyles.SupportsTransparentBackColor | ControlStyles.Selectable, true);
            BackColor = Color.Transparent;
            ForeColor = LabelColor;
            TabStop = true;
            Height = 26;
            Width = 160;
            Cursor = Cursors.Hand;
            Font = new Font("Segoe UI", 11f);
        }

        public bool Checked
        {
            get => _checked;
            set
            {
                if (value == _checked) return;
                _checked = value;
                Invalidate();
                CheckedChanged?.Invoke(this, EventArgs.Empty);
            }
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            var g = e.Graphics;
            g.SmoothingMode = SmoothingMode.AntiAlias;

            int boxY = (Height - BoxSize) / 2;
            var box = new Rectangle(1, boxY, BoxSize, BoxSize);

            using (var b = new SolidBrush(_checked ? AccentColor : BoxColor))
                g.FillRectangle(b, box);
            using (var p = new Pen(_checked ? AccentColor : BoxBorder))
                g.DrawRectangle(p, box);

            if (_checked)
            {
                using (var p = new Pen(CheckColor, 2f))
                {
                    g.DrawLines(p, new[]
                    {
                        new Point(box.Left + 4,  box.Top + 9),
                        new Point(box.Left + 8,  box.Top + 13),
                        new Point(box.Left + 14, box.Top + 5),
                    });
                }
            }

            var textRect = new Rectangle(BoxSize + Gap, 0, Width - BoxSize - Gap, Height);
            TextRenderer.DrawText(g, Text, Font, textRect, LabelColor,
                TextFormatFlags.Left | TextFormatFlags.VerticalCenter | TextFormatFlags.EndEllipsis);

            if (Focused)
            {
                using (var p = new Pen(AccentColor) { DashStyle = DashStyle.Dot })
                    g.DrawRectangle(p, Rectangle.Inflate(box, 2, 2));
            }
        }

        protected override void OnClick(EventArgs e)
        {
            base.OnClick(e);
            Focus();
            Checked = !_checked;
        }

        protected override bool IsInputKey(Keys keyData)
        {
            if (keyData == Keys.Space) return true;
            return base.IsInputKey(keyData);
        }

        protected override void OnKeyDown(KeyEventArgs e)
        {
            base.OnKeyDown(e);
            if (e.KeyCode == Keys.Space)
            {
                Checked = !_checked;
                e.Handled = true;
            }
        }

        protected override void OnGotFocus(EventArgs e)  { base.OnGotFocus(e); Invalidate(); }
        protected override void OnLostFocus(EventArgs e) { base.OnLostFocus(e); Invalidate(); }
        protected override void OnTextChanged(EventArgs e) { base.OnTextChanged(e); Invalidate(); }
    }
}