Modality, part 5: Setting the correct owner for modal UI

Raymond Chen


Here is the very simple fix for

the buggy program we presented last time

void OnChar(HWND hwnd, TCHAR ch, int cRepeat)
  switch (ch) {
  case ' ':
    MessageBox(hwnd, TEXT("Message"), TEXT("Title"), MB_OK);
    if (!IsWindow(hwnd)) MessageBeep(-1);

We have fixed the problem by passing the correct owner window
for the modal UI. Since MessageBox is modal,
it disables the owner while the modal UI is being displayed,
thereby preventing the user from destroying or
changing the owner window’s state when it is not expecting it.

This is why all the shell functions that can potentially display
UI accept a window handle as one of its parameters.
They need to know which window to use as the owner for any
necessary UI dialogs. If you call such functions from a thread
that is hosting UI, you must pass the handle to the window you
want the shell to use as the UI owner. If you pass NULL
(or worse, GetDesktopWindow), you may find yourself
in the same bad state that our buggy sample program demonstrated.

If you are displaying a modal dialog from another modal dialog,
it is important to pass the correct window as the owner for
the second dialog. Specifically, you need to pass the modal dialog
initiating the sub-dialog and not the original frame window.
Here’s a stack diagram illustrating:

  DialogBox(hwndOwner = main window) [dialog 1]
   ... dialog manager ...
     DialogBox(hwndOwner = dialog 1) [dialog 2]

If you mess up and pass the main window handle when
creating the second modal dialog, you will find yourself
back in a situation analogous to what we had last time:
The user can dismiss the first dialog while the second
dialog is up, leaving its stack frames orphaned.

Raymond Chen
Raymond Chen

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