Mini Paint application in c# windows form

 public partial class Form1 : Form
    {
        Rectangle shape;
        Point p1 = new Point();
        Point p2 = new Point();
       
       
       
        public Form1()
        {
            InitializeComponent();
            //this.DoubleBuffered = true;

        }


        protected override void OnMouseDown(MouseEventArgs e)
        {

            shape = new Rectangle(e.X, e.Y, 0, 0);
            p1 = e.Location;
            this.Invalidate();
        }
        protected override void OnMouseMove(MouseEventArgs e)
        {

            if (e.Button == MouseButtons.Left)
            {
                shape = new Rectangle(shape.Left, shape.Top, e.X - shape.Left, e.Y - shape.Top);

                p2 = e.Location;


            }
          this.Invalidate();
        }
        protected override void OnPaint(PaintEventArgs e)
        {
            Pen pen = new Pen(colorDialog1.Color, 1);
            
            Graphics g = this.CreateGraphics();
          
                g.DrawEllipse(pen, shape);
               
        }       
}