阻止窗口关闭的方法：

1. 使关闭按钮无效变灰
 

const int SC_CLOSE = 0xF060;
        const int MF_BYCOMMAND = 0x0;

        [System.Runtime.InteropServices.DllImport("user32.dll")]
        public static extern IntPtr RemoveMenu(IntPtr menuHwnd, int position, int flags);

        [System.Runtime.InteropServices.DllImport("user32.dll")]
        public static extern IntPtr GetSystemMenu(IntPtr hwnd, int bRevert);

        public static void EnableApplicationClose(System.Windows.Forms.Form form, bool val)
        {
            IntPtr menu = GetSystemMenu(form.Handle, val ? 1 : 0);
            if (!val && menu != IntPtr.Zero)
                RemoveMenu(menu, SC_CLOSE, MF_BYCOMMAND);
            else
            {
                // needed when enabling the app close box to force it to redraw.
                form.Enabled = false;
                form.Enabled = true;
            }
        }

2. 重载onclosing事件处理函数
 

Code:
        private void frmStopClose_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (MessageBox.Show("Close Form?", "Remind", MessageBoxButtons.YesNo) != DialogResult.Yes)
            {
                e.Cancel = true;
            }
        }