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using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication3
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        Rectangle rec = new Rectangle(100, 100, 100, 100);
                
        private void pictureBox1_Paint(object sender, PaintEventArgs e)
        {
            Graphics g = e.Graphics;
            Pen p = new Pen(Brushes.Black);

            g.DrawRectangle(p , rec);
            Region regInner = new Region(rec);
            regInner.IsVisible(rec);
        }

        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            if (rec.regInner.IsVisible(e.Location))
            {
                Cursor.Current = Cursors.Cross;
                return;
            }
        }
    }
}                    

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Region regInner = new Region(rec);

if (regInner.IsVisible(e.Location))
{
   Cursor.Current = Cursors.Cross;
}
else
{
   Cursor.Current = Cursors.Default; // zurücksetzen 
}