关闭窗体弹出对话框

        protected override void OnFormClosing(FormClosingEventArgs e)
        {
            if (e.CloseReason == CloseReason.UserClosing)
            {
                if (MessageBox.Show("Are you sure you want to close?", "Close?", MessageBoxButtons.YesNo) == DialogResult.No)
                {
                    e.Cancel = true;
                }
            }

            base.OnFormClosing(e);
        }