.NET (Core and Framework)

private BitmapCollection layers;
private int currentLayer;
private Size size;
 
private PointCollection points;
 
private Pen currentPen;
 
OnMouseDown(MouseEventArgs e)
{
  // left mouse button down
  if( 0 != (e.Button & MouseButtons.Left) ) 
  {
    points.Clear();
    points.Add( new Point(e.X, e.Y) );
  
    Rectangle rectToInvalidate = new Rectangle(
      e.X - (int) currentPen.Width, e.Y - (int) currentPen.Width,
      ((int) currentPen.Width) * 2, ((int) currentPen.Width) * 2
    );
  
    Invalidate( rectToInvalidate );
  }
}
 
OnMouseMove( MouseEventArgs e )
{
  // left mouse button down
  if( 0 != (e.Button & MouseButtons.Left) ) 
  {
    Point newPoint = new Point(e.X, e.Y);
    points.Add( newPoint );
 
    Rectangle rectToInvalidate = RectFromTwoPoints( 
      newPoint, points[points.Count - 2], (int) currentPen.Width);
 
    Invalidate( rectToInvalidate );
  }
}
 
OnMouseUp( MouseEventArgs e )
{
  // left mouse button down
  if( 0 != (e.Button & MouseButtons.Left) ) 
  {
    using(Graphics g = Graphics.FromBitmap( layers[currentLayer] );
    {
      Point [] pointsArray = points.ToArray(typeof(Point[]));
  
      g.DrawLines( currentPen, pointsArray );
      
      points.Clear();
    }
  }
}
 
OnPaint( PaintEventArgs e )
{
  for(int = 0; i < layers.Count; i++ )
  {
    // Draw layer i, where 0 is the bottom layer
    // and layers.Count -1 is the top layer
 
    if( i == currentLayer && points.Count > 0 )
    {
      Point [] pointsArray = points.ToArray(typeof(Point[]));
  
      g.DrawLines( currentPen, pointsArray );
    }
  }
}
 
Rectangle RectFromTwoPoints( Point p1, Point p2, int penWidth )
{
  int left, top, right, bottom;
 
  if( p1.X < p2.X )
  {
    left = p1.X;
    right = p2.X;
  }
  else if( p1.X > p2.X )
  {
    left = p2.X;
    right = p1.X;
  }
  else // p1.X == p2.X
  {
    left = right = p1.X;
  }
 
  if( p1.Y < p2.Y )
  {
    top = p1.Y;
    bottom = p2.Y;
  }
  else if( p1.Y > p2.Y )
  {
    top = p2.Y;
    bottom = p1.Y;
  }
  else // p1.Y == p2.Y
  {
    top = bottom = p1.Y;
  }
 
  top -= penWidth;
  left -= penWidth;
  right += penWidth;
  bottom += penWidth;
 
  return Rectangle.FromLTRB( left, top, right, bottom );
}