public Form1()
        {
            InitializeComponent();
        }
        List<Rectangle> rects=new List<Rectangle>() ;
        
        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            Graphics g = this.CreateGraphics();
            Rectangle rect = new Rectangle(e.X,e.Y,100,100);
            g.DrawRectangle(Pens.Red, rect);
            rects.Add(rect);
        }

        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                if (rects.Count() > 0)
                {
                    Graphics g = this.CreateGraphics();
                    Rectangle rect = rects.Last();
                    int x = rect.X;
                    int y = rect.Y;
                    rect = new Rectangle(x, y, e.X - x, e.Y - y);
                    g.DrawRectangle(Pens.Red, rect);
                    //this.Refresh();
                    //rects.Add(rect);
                    
                }
            }
        }