Code (C++)

        /// <summary>
        /// Called when the mouse wheel is used over a diagram.
        /// Default behavior is to zoom in or out of the diagram.
        /// The zooming is to the center of the visible screen.
        /// </summary>
        /// <param name="sender">Not used</param>
        /// <param name="e">Information about the mouse wheel event</param>
        private void diagramView_MouseWheel(object sender, MouseEventArgs e)
        {
            if (e.Delta > 0)
            {
                ZoomToCenter(diagramView.ZoomFactor + 10);
            }
            else if (e.Delta < 0)
            {
                ZoomToCenter(diagramView.ZoomFactor - 10);
            }
        } 

Code

diagramView.MouseWheel += diagramView_MouseWheel;