C# Form1 & Hình Chữ Nhật, Hình Hộp

using System;

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

namespace WindowsFormsApp3
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void checkBox1_CheckedChanged(object sender, EventArgs e)
        {
            if(checkBox1.Checked==true)
            {
                label3.Enabled = true;
                textBox3.Enabled = true;
            }
            else
            {
                label3.Enabled = false;
                textBox3.Enabled = false;
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            int cd=int.Parse(textBox1.Text);
            int cr = int.Parse(textBox2.Text);
            if(checkBox1.Checked==true)//hinh hop
            {
                int cc = int.Parse(textBox3.Text);
                HH a = new HH(cd, cr, cc);
                label4.Text= a.xuatTT();
            }
            else
            {
                HCN a = new HCN(cd, cr);
                label4.Text = a.xuatTT();
            }
        }
    }
}