AreaSelector

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;
 
namespace WindowsFormsApplication241
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
 
            new AreaSelector(this);
        }
    }
 
    class AreaSelector:IDisposable
    {
        private Control parent;
 
        public readonly List<Point> Points = new List<Point>();
        private Point mousePoint;
 
        public AreaSelector(Control parent)
        {
            this.parent = parent;
            parent.MouseDown += parent_MouseDown;
            parent.MouseMove += parent_MouseMove;
            parent.Paint += parent_Paint;
        }
 
        void parent_MouseMove(object sender, MouseEventArgs e)
        {
            mousePoint = e.Location;
            parent.Invalidate();
        }
 
        void parent_Paint(object sender, PaintEventArgs e)
        {
            e.Graphics.SmoothingMode = SmoothingMode.HighQuality;
 
            var path = new GraphicsPath();
            if (Points.Count > 0)
            {
                path.AddLines(Points.ToArray());
                path.AddLines(new Point[] {mousePoint});
                path.CloseFigure();
                e.Graphics.DrawPath(Pens.Black, path);
            }
        }
 
        void parent_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button != MouseButtons.Left)
                return;
 
            Points.Add(e.Location);
            parent.Invalidate();
        }
 
        public void Dispose()
        {
            parent.MouseDown -= parent_MouseDown;
            parent.MouseMove -= parent_MouseMove;
            parent.Paint -= parent_Paint;
        }
    }
}