width = 2
height = 2
depth = 2

width = 2
height = 20
depth = 2

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;
using System.Drawing.Drawing2D;


namespace dragit
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            DoubleBuffered = true;
        }
        private Point selectionStart;
        private Point selectionEnd;
        private Rectangle selection;
        private bool mouseDown;

        private void GetSelectedTextBoxes()
        {
            List<CheckBox> selected = new List<CheckBox>();

            foreach (Control c in Controls)
            {
                if (c is CheckBox)
                {
                    if (selection.IntersectsWith(c.Bounds))
                    {
                        selected.Add((CheckBox)c);

                        ((CheckBox)c).Checked = true;

                    }
                }
            }

            // Replace with your input box
            // MessageBox.Show("You selected " + selected.Count + " textbox controls.");

        }

        protected override void OnMouseDown(MouseEventArgs e)
        {
            selectionStart = PointToClient(MousePosition);
            mouseDown = true;
        }

        protected override void OnMouseUp(MouseEventArgs e)
        {
            mouseDown = false;

            // SetSelectionRect();
            Invalidate();

        }

        protected override void OnMouseMove(MouseEventArgs e)
        {
            if (!mouseDown)
            {
                return;
            }

            selectionEnd = PointToClient(MousePosition);
            SetSelectionRect();

            Invalidate();
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);

            if (mouseDown)
            {
                using (Pen pen = new Pen(Color.Black, 1F))
                {
                    pen.DashStyle = DashStyle.Dash;
                    e.Graphics.DrawRectangle(pen, selection);
                }
            }
        }

        private void SetSelectionRect()
        {
            int x, y;
            int width, height;

            x = selectionStart.X > selectionEnd.X ? selectionEnd.X : selectionStart.X;
            y = selectionStart.Y > selectionEnd.Y ? selectionEnd.Y : selectionStart.Y;

            width = selectionStart.X > selectionEnd.X ? selectionStart.X - selectionEnd.X : selectionEnd.X - selectionStart.X;
            height = selectionStart.Y > selectionEnd.Y ? selectionStart.Y - selectionEnd.Y : selectionEnd.Y - selectionStart.Y;

            selection = new Rectangle(x, y, width, height);

            GetSelectedTextBoxes();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            foreach (Control c in Controls)
            {
                if (c is CheckBox)
                {

                    ((CheckBox)c).Checked = false;

                }
            }
        }

    }
}