Drawing Rectangular with Mouse

namespace Zau_DrawRectWithMouse
{
    public partial class Form1 : Form
    {
        Bitmap bmap;
        Graphics g;



        Rectangle rect;

        public Form1()
        {
            InitializeComponent();
            // to use the cross "+" cursor
            this.Cursor = System.Windows.Forms.Cursors.Cross;
            // to reduce flicker
            this.DoubleBuffered = true;

            bmap = new Bitmap(pictureBox1.Width, pictureBox1.Height);
            g = Graphics.FromImage(bmap);
        }

        private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
        {
            // "e.X" and "e.Y" are used to get MousePositionX and MousePositionY
            rect = new Rectangle(e.X, e.Y, 0, 0);
            this.Invalidate();
        }

        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            // This makes sure that the left mouse button is pressed.
            if (e.Button == MouseButtons.Left)
            {
                // Draws the rectangle as the mouse moves
                rect = new Rectangle(rect.Left, rect.Top, e.X - rect.Left, e.Y - rect.Top);

            }
            this.Invalidate();
        }

        private void pictureBox1_Paint(object sender, PaintEventArgs e)
        {
            //using (Pen pen = new Pen(Color.White, 3))
            //{
            //    e.Graphics.DrawRectangle(pen, rect);
            //    pictureBox1.Image = bmap;
            //}
        }

        private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                Pen pen = new Pen(Color.White, 3);
                g.DrawRectangle(pen, rect);
                pictureBox1.Image = bmap;
            }

        }

        private void btnClearCanvas_Click(object sender, EventArgs e)
        {
            bmap = new Bitmap(pictureBox1.Width, pictureBox1.Height);
            g = Graphics.FromImage(bmap);
            pictureBox1.Image = bmap;
        }
    }
}