消失的FormClosing / FormClosed事件

Question

Following code snippet may look weird, but it is a very simple method to reproduce the problem of a complicated interaction with a different application.

The example application has two forms, both forms register FormClosing and FormClosed events, just doing a Debug.WriteLine there.

The Main function does:

[STAThread]
static void Main()
{
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    Form2 form2 = new Form2();
    form2.Show();
    Application.Run(new Form1());
}

When I close Form2, I receive the events as expected. When I close Form1, the application exits and Form2 gets closed too. But: the events of Form2 do not occur.

I overrode the WndProc of Form2. When I close Form2, I receive

15.10.2012 10:25:04 WndProc - WM_CLOSE
15.10.2012 10:25:04 OnClosing
15.10.2012 10:25:04 Form2_FormClosing
15.10.2012 10:25:04 OnClosed
15.10.2012 10:25:04 Form2_FormClosed
...
15.10.2012 10:25:04 WndProc - WM_DESTROY
...
15.10.2012 10:25:04 WndProc - WM_NCDESTROY

When I close Form1, Form2 does not receive a WM_CLOSE. But WM_DESTROY and WM_NCDESTROY are received.

Edit: A different way of getting this strange behavior is as follows: The project has three forms, Form1 and Form2 have each a button for opening the next form. In the button click event of form2, do:

Form3 frm3 = new Form3();
frm3.Parent = null;
frm3.Show(null);

Again, register event handlers as described above. When closing form2, the application stays running (form1 is still shown), but also form3 gets closed, and no WM_CLOSE event received. Note that I already set both Parent and Owner of the form to null.

What is the reason for that strange behavior?

Answer 1

WM_CLOSE message is sent to have an option of canceling closing. See this flowchart:

So, there is no need in this message, when window is about to be destroyed anyway. This is behavior by design. When you are closing main form of application, Application.ThreadExit method is called. It described as:

Exits the message loop on the current thread and closes all windows on the thread.

If you want all opened forms to receive WM_CLOSE message, you need to use Application.Exit instead:

Informs all message pumps that they must terminate, and then closes all application windows after the messages have been processed.

Thus simply subscribe to Form1_FormClosing event and inform all opened windows about closing:

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
    Application.Exit();
}