Code

    private void OnMouseWheel(object sender, System.Windows.Forms.MouseEventArgs e)
    {
      // Update the drawing based upon the mouse wheel scrolling.
      if (_theControlKeyPressed)
      {
        int factorToIncrese = e.Delta * SystemInformation.MouseWheelScrollLines / 120;
        diagramView.ZoomFactor += factorToIncrese;
        diagramView.Invalidate();
      }
      else
      {
        int factorToIncrese = e.Delta * SystemInformation.MouseWheelScrollLines / 120;
        diagramView.ScrollY -= factorToIncrese;
        if (diagramView.ScrollY <= 0)
          diagramView.ScrollY = 0;
      }
    }