窗口样式中的ControlBox选为False就可以去掉右上角的叉
（2）重写关闭事件
private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
DialogResult result;
result = MessageBox.Show("确定退出吗？", "退出", MessageBoxButtons.OKCancel, MessageBoxIcon.Question);
if (result == DialogResult.OK)
{
Application.ExitThread();
}
else
{
e.Cancel = true;
}
}