Principal.cs

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;

namespace R802DeteccionClickFigura
{
    public partial class Principal : Form
    {
        private GraphicsPath path;
        private Rectangle rectangulo;

        private bool dentroDelPath = false;
        private bool dentroDelRectangulo = false;

        Brush rosado = Brushes.HotPink;
        Brush azulClaro = Brushes.LightBlue;

        public Principal()
        {
            InitializeComponent();
        }

        private void Principal_Load(object sender, EventArgs e)
        {
            path = new GraphicsPath();
            path.AddEllipse(10, 10, 100, 60);
            path.AddCurve(new Point[] {new Point(50, 50),
                new Point(10,33), new Point(80,43)});
            path.AddLine(50, 120, 250, 80);
            path.AddLine(120, 40, 110, 50);
            path.CloseFigure();

            rectangulo = new Rectangle(100, 170, 220, 120);
        }

        private void Principal_Paint(object sender, PaintEventArgs e)
        {
            Graphics g = e.Graphics;

            if (dentroDelPath)
            {
                g.FillPath(rosado, path);
                g.FillRectangle(azulClaro, rectangulo);
            }
            else if (dentroDelRectangulo)
            {
                g.FillRectangle(rosado, rectangulo);
                g.FillPath(azulClaro, path);
            }
            else
            {
                g.FillPath(azulClaro, path);
                g.FillRectangle(azulClaro, rectangulo);
            }

            g.DrawPath(Pens.Black, path);
            g.DrawRectangle(Pens.Black, rectangulo);
        }

        private void Principal_MouseMove(object sender, MouseEventArgs e)
        {
            using (Graphics g = this.CreateGraphics())
            {
                if (rectangulo.Contains(e.X, e.Y))
                {
                    if (!dentroDelRectangulo)
                    {
                        dentroDelRectangulo = true;

                        g.FillRectangle(rosado, rectangulo);
                        g.DrawRectangle(Pens.Black, rectangulo);
                    }
                }
                else if (dentroDelRectangulo)
                {
                    dentroDelRectangulo = false;

                    g.FillRectangle(azulClaro, rectangulo);
                    g.DrawRectangle(Pens.Black, rectangulo);
                }

                if (path.IsVisible(e.X, e.Y))
                {
                    if (!dentroDelPath)
                    {
                        dentroDelPath = true;

                        g.FillPath(rosado, path);
                        g.DrawPath(Pens.Black, path);
                    }
                }
                else if (dentroDelPath)
                {
                    dentroDelPath = false;

                    g.FillPath(azulClaro, path);
                    g.DrawPath(Pens.Black, path);
                }
            }
        }
    }
}