Tengo problemas para dibujar una línea dentro de un cuadro de grupo en un formulario de ventanas simple.

Aquí está mi código:

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();                        
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);            
            DrawLShapeLine(groupBox1.CreateGraphics(), 10, 10, 20, 40);
        }

        public void DrawLShapeLine(System.Drawing.Graphics g, int intMarginLeft, int intMarginTop, int intWidth, int intHeight)
        {
            Pen myPen = new Pen(Color.Black);
            myPen.Width = 2;
            // Create array of points that define lines to draw.
            int marginleft = intMarginLeft;
            int marginTop = intMarginTop;
            int width = intWidth;
            int height = intHeight;
            int arrowSize = 3;
            Point[] points =
             {
                new Point(marginleft, marginTop),
                new Point(marginleft, height + marginTop),
                new Point(marginleft + width, marginTop + height),
                // Arrow
                new Point(marginleft + width - arrowSize, marginTop + height - arrowSize),
                new Point(marginleft + width - arrowSize, marginTop + height + arrowSize),
                new Point(marginleft + width, marginTop + height)
             };

            g.DrawLines(myPen, points);
        }
    }

Si adjunto el método DrawLShapeLine a un evento de clic de botón, se dibuja bien, pero no se dibuja en la carga del formulario.

Por favor aconséjame.