回答編集履歴

  `FillRectangle`相当になってしまいますが、こんなのはどうでしょう？

 

  図形の保存やヒットテスト等、面倒なことをパネルにお任せしてしまいます（図形を`GraphicsPath`にするのがすこし手間ですが）

 

  using System;

  using System.Drawing;

  using System.Drawing.Drawing2D;

  using System.Windows.Forms;

 

  namespace Questions260581

  {

  public partial class Form1 : Form

  {

  public Form1()

  InitializeComponent();

  ClientSize = new Size(1000, 600);

 

  var left = 2;

  var top = 2;

  var width = 150;

  var height = 150;

 

  left += 4;

 

  var panel = new Panel

  {

  Top = top,

  Left = left,

  Width = width,

  Height = height,

  BackColor = Color.Blue,

  };

  panel.MouseEnter += Panel_MouseEnter;

  panel.MouseLeave += Panel_MouseLeave;

  Controls.Add(panel);

 

  left = 800;

  top = 2;

 

  var path = new GraphicsPath(FillMode.Winding);

  path.AddLines(StarPoints(5, new Rectangle(0, 0, width, height)));

  var region = new Region(path);

 

  left -= 4;

 

  var panel = new Panel

  {

  Top = top,

  Left = left,

  Width = width,

  Height = height,

  BackColor = Color.Blue,

  Region = region,

  };

 

  panel.MouseEnter += Panel_MouseEnter;

  panel.MouseLeave += Panel_MouseLeave;

  Controls.Add(panel);

 

  private void Panel_MouseEnter(object sender, EventArgs e)

  if(sender is Panel p)

  p.BackColor = Color.Red;

  p.BringToFront();

 

  private void Panel_MouseLeave(object sender, EventArgs e)

  if(sender is Panel p)

  p.BackColor = Color.Blue;

 

  //http://csharphelper.com/blog/2014/08/draw-a-star-in-c/

  private PointF[] StarPoints(int num_points, Rectangle bounds)

  var pts = new PointF[num_points];

 

  double rx = bounds.Width / 2;

  double ry = bounds.Height / 2;

  var cx = bounds.X + rx;

  var cy = bounds.Y + ry;

 

  var theta = -Math.PI / 2;

  var dtheta = 4 * Math.PI / num_points;

  for(var i = 0; i < num_points; i++)

  pts[i] = new PointF(

  (float)(cx + rx * Math.Cos(theta)),

  (float)(cy + ry * Math.Sin(theta)));

  theta += dtheta;

 

  return pts;

  }

  }

  ```

  

 

  ---

 

  追記 雑な自前描画版

  using System.Collections.Generic;

  using System.Drawing;

  using System.Drawing.Drawing2D;

  using System.Windows.Forms;

 

  namespace Questions260581

  {

  public partial class Form1 : Form

  {

  private readonly List<GraphicsPath> paths = new List<GraphicsPath>();

  private GraphicsPath selected = new GraphicsPath(); // nullチェックが面倒なのでカラのパス

 

  public Form1()

  InitializeComponent();

  ClientSize = new Size(1000, 600);

  MouseMove += Form1_MouseMove;

 

  var left = 2;

  var top = 2;

  var width = 150;

  var height = 150;

 

  left += 4;

 

  var path = new GraphicsPath();

  path.AddRectangle(new Rectangle(left, top, width, height));

  paths.Add(path);

 

  left = 800;

  top = 2;

  var lines = new PointF[] // StarPoints()が閉じてくれていないので手打ち^^;

  new PointF(75f, 0f),

  new PointF(119.0839f, 135.6763f),

  new PointF(3.670761f, 51.82373f),

  new PointF(146.3292f, 51.82373f),

  new PointF(30.91611f, 135.6763f),

  new PointF(75f, 0f),

  };

  left -= 4;

 

  var path = new GraphicsPath();

  path.AddLines(lines);

  var matrix = new Matrix();

  matrix.Translate(left, top, MatrixOrder.Append);

  path.Transform(matrix);

  paths.Add(path);

 

  private void Form1_MouseMove(object sender, MouseEventArgs e)

  foreach(var path in paths)

  if(!path.IsVisible(e.Location)) continue;

  if(path == selected) return;

 

  using(var g = CreateGraphics())

  {

  g.DrawPath(Pens.Blue, selected);

  selected = path;

  g.DrawPath(Pens.Red, selected);

  }

  return;

 

  using(var g = CreateGraphics())

  g.DrawPath(Pens.Blue, selected);

 

  selected = new GraphicsPath(); // 選択なし nullチェックが面倒なのでカラのパス

 

  protected override void OnPaint(PaintEventArgs e)

  base.OnPaint(e);

 

  foreach(var path in paths)

  e.Graphics.DrawPath(Pens.Blue, path);

  }

  }

  ```