Question

If one needs some sort of state in a dialog function, it seems you need some sort of static variable. For example, the hook procedure for GetOpenFileName provides the dialog function with the address of the OPENFILENAME structure on its WM_INITDIALOG call but not on any subsequent calls and the OPENFILENAME structure contains a place for lCustData which could be used to pass information between the dialog procedure and the GetOpenFileName caller. If the dialog function needs either of these states, it seems the only alternative is a static or global variable.

Answer 1

You can use SetProp in WM_INITDIALOG like this KB recommends, or SetWindowLongPtr(..,GWLP_USERDATA) if you have custom controls (template)