public Form6()
        {
            InitializeComponent();
            foreach (CheckBox ck in groupBox1.Controls)
            {
                 ck.CheckedChanged += Ck_CheckedChanged;
            }
        }
        private void checkBox1_Click(object sender, EventArgs e)
        {
            if (checkBox1.CheckState == CheckState.Checked)
            {
                foreach (CheckBox ck in groupBox1.Controls)
                    ck.Checked = true;
                checkBox1.Text = "反选";
            }
            else
            {
                foreach (CheckBox ck in groupBox1.Controls)
                    ck.Checked = false;
                checkBox1.Text = "全选";
            }
        }

        private void Ck_CheckedChanged(object sender, EventArgs e)
        {
            CheckBox c = sender as CheckBox;
            if (c.Checked == true)
            {
                foreach (CheckBox ch in groupBox1.Controls)
                {
                    if (ch.Checked == false)
                        return;
                }
                checkBox1.Checked = true;
                checkBox1.Text = "反选";
            }
            else
            {
                checkBox1.Checked = false;
                checkBox1.Text = "全选";
            }
        }