[Windows Form] show overlay crosshair

CrosshairForm.cs

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

namespace crosshair
{
    class CrosshairForm : Form
    {
        public CrosshairForm()
        {
            // Form 設定
            string iconPath = @"./xxx.ico"; //iconファイルパス
            Icon icon = new Icon(iconPath, 128, 128);
            this.Icon = icon;
            StartPosition = FormStartPosition.CenterScreen;
            TopMost = true;
            FormBorderStyle = FormBorderStyle.None;
            TransparencyKey = BackColor;
            Opacity = 0.2;

            // クロスヘア
            Label lbl = new Label
            {
                AutoSize = false,
                TextAlign = ContentAlignment.MiddleCenter,
                Dock = DockStyle.Fill,
            };
            lbl.Font = new Font(lbl.Name, 30f, lbl.Font.Style);
            lbl.ForeColor = Color.White;
            lbl.Text = "◯";

            Controls.Add(lbl);
        }

        /// <summary>
        /// クリックイベントの透過
        /// </summary>
        protected override CreateParams CreateParams
        {
            get
            {
                CreateParams cp = base.CreateParams;
                cp.ExStyle |= 0x00000020;
                return cp;
            }
        }
    }
}