它确实如L.B所说：在窗体类中创建一个重写。它可以放在类中的任何位置
在关闭时无效（CancelEventArgs e）数据。Windows = -1;//假设该变量在那时仍然存在}

以下是您的答案：

private void Form1_MdiChildActivate(object sender, EventArgs e)
{
    //form is new mdi child
    Form form = this.MdiChildren.Last();
    form.FormClosed += SenderForm_Close;
}

private void SenderForm_Close(object sender, EventArgs e)
{
    //form is new closed mdi child
    Form form = sender as Form;
}