Can't detect when Windows Font Size has changed C++ MFC

[EDIT after re-read] I'm almost positive that changing to "Large fonts" does not cause a DPI change, rather it's a theme setting. You should be able to verify by applying the "Large fonts" change and then opening the advanced display properties where the DPI setting lives, it should have remained at 96dpi.

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DPI change is supposed to require a reboot. Maybe the setting hasn't propagated to a place where GetDeviceCaps can retrieve it?

Maybe try changing a non-reboot-requiring setting (resolution perhaps) and then see if you can detect the change. If you can, your answer is probably that you can't detect DPI change until after reboot.

When you call GetDeviceCaps() on the Desktop DC, are you perhaps using a DC that might be cached by MFC, and therefore contains out-of-date information? Are you making the GetDeviceCaps() call synchronously from inside your OnSettingsChange handler? I could see how either or both of these things might get you an out of date version of DPI.

Raymond Chen wrote about this and his solution looked like this (Note that I've added :: operators to avoid calling the MFC wrappers of the APIs):

int GetScreenDPI()
{
  HDC hdcScreen = ::GetDC(NULL);
  int iDPI = -1; // assume failure
  if (hdcScreen) {
    iDPI = ::GetDeviceCaps(hdcScreen, LOGPIXELSX);
    ::ReleaseDC(NULL, hdcScreen);
  }
  return iDPI;
}

I have a hunch WM_THEMECHANGED will take care of you. It doesn't have any hinting about what changed, though. You'll have to use OpenThemeData and cache initial settings, then compare every time you get the message.

You probably don't need to care what changed though, can't you have a general-purpose layout routine that adjusts your form/dialog/whatever by taking everything into account and assumes starting from scratch?

What problem are you trying to solve?