show().和ShowDialog()有什么区别。为什么这个事件只能在SHOW里用？

        private void btnAdd_Click(object sender, EventArgs e)
        {
            //禁用当前窗口
            this.Enabled = false;
            frmAdd add = new frmAdd();
            add.Show();

           add.FormClosing += new FormClosingEventHandler(add_FormClosing);
        }

        private void add_FormClosing(object sender, FormClosingEventArgs e)
        {
            this.Enabled = true;
            this.bind();
        }