Вывод графики на экран, поверх всех окон.

using System.Runtime.InteropServices;
...
[DllImport("user32.dll", SetLastError = true)]
static extern IntPtr GetDC(IntPtr hWnd);
 
[DllImport("user32.dll", SetLastError = true)]
static extern int ReleaseDC(IntPtr hWnd, IntPtr hDC);
...
IntPtr hDC = GetDC(IntPtr.Zero);
using (Graphics g = Graphics.FromHdc(hDC)) {
    g.DrawString("Текст", this.Font, Brushes.Red, 300, 400);
}
ReleaseDC(IntPtr.Zero, hDC);

g.DrawString("Текст", this.Font, Brushes.Red, 300, 400);

g.DrawLine(Pens.Red, 0, 0, 500, 500);

[DllImport("user32.dll")]
        static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);
 
        [DllImport("user32.dll", SetLastError = true)]
        static extern int GetWindowLong(IntPtr hWnd, int nIndex);
 
        private const int ExStyle = -20;
 
        private const int Transparent = 0x20;
 
        private const int Layered = 0x80000;
 
        private int initialStyle;
 
        public Form1()
        {
            InitializeComponent();
            initialStyle = GetWindowLong(Handle, ExStyle);
            TransparencyKey = BackColor;
            SetWindowLong(Handle, ExStyle, initialStyle | Layered | Transparent);
            StartPosition = FormStartPosition.CenterScreen;
            TopMost = true;
            ShowIcon = false;
            ShowInTaskbar = false;
        }
 
        protected override void OnPaint(PaintEventArgs e)
        {
            Font f = new Font("Microsoft Sans Serif", 30f);
            e.Graphics.TextRenderingHint = System.Drawing.Text.TextRenderingHint.ClearTypeGridFit;
;
            e.Graphics.DrawString("HELLO WORLD!", f, new SolidBrush(Color.Red), new PointF(0, 0) );
        }
        const int WM_NCHITTEST = 0x0084;
        const int HTTRANSPARENT = -1;
 
        protected override void WndProc(ref Message m)
        {
            if (m.Msg == WM_NCHITTEST)
            {
                m.Result = (IntPtr)HTTRANSPARENT;
                return;
            }
            base.WndProc(ref m);
        }