Thay đổi trạng thái form c# (WindowState)

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 _1_start
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnclose_Click(object sender, EventArgs e)
        {
            Close();
        }

        private void btnmini_Click(object sender, EventArgs e)
        {
            this.WindowState = FormWindowState.Minimized;
        }

        private void btnmax_Click(object sender, EventArgs e)
        {
            this.WindowState = FormWindowState.Maximized;
        }

        private void btntb_Click(object sender, EventArgs e)
        {
            this.WindowState = FormWindowState.Normal;
        }
    }
}