//:. Occurs when the mouse pointer is over the control and a mouse button is released
        private void PictureBoxPreview_MouseUp(object sender, MouseEventArgs e)
        {
            this.PictureBoxPreview.Cursor = Cursors.Arrow;

            if ( this.bFlagMouseDown )
            {
                this.bFlagMouseDown = false;
               
                //Recupero il contesto grafico dall'immagine nella picture box
                Graphics g = Graphics.FromImage( this.PictureBoxPreview.Image );

                // Creazione pen (colore nero).
                Pen blackPen = new Pen( Color.Black, 2 );

                // Posizione e dimensione ellisse (cerchio)
                int nDistanceBetweenPoints = (int)Math.Sqrt( (Math.Pow((this.StartX - this.EndX), 2) + Math.Pow((this.StartY - this.EndY), 2)) );
                
                
                int width = 2 * nDistanceBetweenPoints;
                int height = 2 * nDistanceBetweenPoints;

                // Draw ellipse to screen.
                g.DrawEllipse( blackPen, this.StartX, this.StartY, width, height );

              }
        }

       

        //:. Occurs when the mouse pointer leaves the control
        private void PictureBoxPreview_MouseLeave(object sender, EventArgs e)
        {
            label2.Text = sender.GetType().ToString() + ": MouseLeave";
            this.bFlagMouseEnter = false;
        }
    }