C# 退出对话框 DialogResult 用法

下面的代码可以防止对话框弹出二次的尴尬。\

  /// <summary>
        /// 点击关闭后，停止程序
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void indexForm_FormClosed(object sender, FormClosedEventArgs e)
        {
            Application.Exit();
        }
        /// <summary>
        /// 点击关闭时询问
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void indexForm_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (MessageBox.Show("是否退出系统", "提示", MessageBoxButtons.YesNo, MessageBoxIcon.Question) == DialogResult.No)
            {
                e.Cancel = true;
            }
        }
        /// <summary>
        /// 退出系统
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void 退出系统ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            if (MessageBox.Show("是否退出系统", "提示", MessageBoxButtons.YesNo, MessageBoxIcon.Question) == DialogResult.Yes)
            {
               Application.ExitThread();
            }
            else
            {
                return;
            }
        }