第二个窗体是窗体/类本身的示例，因此除非您将其设置为MDI应用程序，否则第二个窗体将不会接收Closing或Closed事件，因为程序正在退出。
您可以从Form 1的关闭事件点击进入第二个窗体FormClosing事件，这是许多方法之一：

public Form1()
{
    InitializeComponent();
    frm2.FormClosing += new FormClosingEventHandler(form2_FormClosing);
    frm2.Show();
}

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
    form2_FormClosing(null, e);
}

private void form2_FormClosing(object? sender, FormClosingEventArgs e)
{
   // This will only get called "directly" when the second form is 
   // closed otherwise it will always be called from Form1_FormClosing.
}