How can a child window respond to a change in its parent

Question

In a Win32 app is there a Windows message or some other notification that will get sent to a child window when it is placed into a different parent

Answer 1

This is easy to test in a Windows Forms app. This is what I saw:

msg=0x18 (WM_SHOWWINDOW) hwnd=0x60c60 wparam=0x1 lparam=0x0 result=0x0
msg=0x46 (WM_WINDOWPOSCHANGING) hwnd=0x60c60 wparam=0x0 lparam=0x563e01c result=0x0
msg=0x85 (WM_NCPAINT) hwnd=0x60c60 wparam=0x1 lparam=0x0 result=0x0
msg=0x14 (WM_ERASEBKGND) hwnd=0x60c60 wparam=0xffffffff930119e8 lparam=0x0 result=0x0
msg=0x47 (WM_WINDOWPOSCHANGED) hwnd=0x60c60 wparam=0x0 lparam=0x563e01c result=0x0
msg=0xf (WM_PAINT) hwnd=0x60c60 wparam=0x0 lparam=0x0 result=0x0
msg=0xe (WM_GETTEXTLENGTH) hwnd=0x60c60 wparam=0x0 lparam=0x0 result=0x0
msg=0xd (WM_GETTEXT) hwnd=0x60c60 wparam=0x6 lparam=0x3fd7928 result=0x0

WM_SHOWWINDOW would be a good time to check if the parent changed. Not 100% sure if this is a side effect of the WF code taking care of the changed parent, the odds are fairly high. There is otherwise no dedicated message for it, the assumption is that the program already knows since it called SetParent or SetWindowLongPtr explicitly.