Snippets...

PropEnumProc.REMOVE: label .hWnd dword at esp+4 label .lpszString dword at esp+8 label .hData dword at esp+12 pop [.hData-4] call [RemoveProp] retn

http://support.microsoft.com/kb/131462

Code:

; Preventing FNERR_BUFFERTOOSMALL with multiple file selection in Open File
; common dialog. http://support.microsoft.com/kb/131462
;
; Although it is doubtful that a single application would have concurrent
; multiple Open File dialogs selecting multiple files; multiple instances of
; the same application could be open and using the same code. So, Microsoft
; advises the use of a control property to preserve the OPENFILENAME pointer.
;
; Microsoft indicates to attach property to parent control, but no reason it
; can't be attached to this default or custom child control (simplifying the
; code slightly). Logically, the structure belongs to the parent, and so does
; the child control.
OFNHookProc:
      label .hWnd     dword at esp+4*1
    label .uMsg     dword at esp+4*2
    label .wParam   dword at esp+4*3
    label .lParam   dword at esp+4*4

        cmp [.uMsg],WM_NOTIFY
       je .WM_NOTIFY
       cmp [.uMsg],WM_DESTROY
      je .WM_DESTROY
      cmp [.uMsg],WM_INITDIALOG
   je .WM_INITDIALOG

       xor eax,eax
 retn 16

.WM_INITDIALOG:
  invoke SetProp,[.hWnd+8],.T_OFN,[.lParam]
   xor eax,eax
 retn 16

.WM_DESTROY:
     invoke RemoveProp,[.hWnd+4],.T_OFN
.WM_0:        xor eax,eax
 retn 16

.WM_NOTIFY:
      mov eax,[.lParam]
   cmp [eax+NMHDR.code],CDN_SELCHANGE
  jne .WM_0

       invoke GetParent,[.hWnd]
    ; returns characters needed (undocumented?) 1+
      invoke SendMessage,eax,CDM_GETSPEC,0,0
      add eax,MAX_PATH
    push eax
    invoke GetProp,[.hWnd+8],.T_OFN
     pop edx
     test eax,eax
        je .WM_0
    cmp [eax+OPENFILENAME.nMaxFile],edx
 jnc .WM_0

       pushad
      xchg eax,edi
        mov esi,edx
 invoke GetProcessHeap
       xchg eax,ebx

; this really isn't needed as HeapFree error is ignored and passing a NULL
; pointer goes right through.
;   cmp [eax+OPENFILENAME.lpstrFile],0
; je @F
       invoke HeapFree,ebx,0,[edi+OPENFILENAME.lpstrFile]
@@:   lea ebp,[esi*2] ; UNICODE: characters to bytes needed
   invoke HeapAlloc,ebx,HEAP_ZERO_MEMORY,ebp
   test eax,eax
        je @F
       mov [edi+OPENFILENAME.lpstrFile],eax
        mov [edi+OPENFILENAME.nMaxFile],esi
@@:  popad
       xor eax,eax
 retn 16

.T_OFN TCHAR "OFN",0

Allocate space on the heap for OPENFILENAME.lpstrFile prior to calling GetOpenFileName as that pointer cannot be null. Also, note that WM_NOTIFY messages can be sent prior to property being created - which surprised me. It's possible to further simplify the WM_NOTIFY code by increasing the size of OPENFILENAME structure to hold parent window and heap handles (to be initialized in WM_INITDIALOG handler). Only mentioned for the sake of completeness.

FILE.TEMPLATE TCHAR "%07u%07u%07u.log",0 push esp push esp invoke GetSystemTimeAsFileTime,esp mov eax,[esp+4] xor edx,edx mov ecx,10000000 div ecx xchg eax,[esp] div ecx xchg edx,[esp] div ecx push edx push eax invoke wsprintf,FILE.PATHNAME,FILE.TEMPLATE add esp,24