lpfn rb 1000h
txt_filter db 'Text files',0,'*.TXT',0
ofn OPENFILENAME
...
mov [ofn.lpstrFile],lpfn
mov [lpfn],0
mov [ofn.lpstrFilter],txt_filter
mov [ofn.Flags],OFN_EXPLORER+OFN_FILEMUSTEXIST+OFN_HIDEREADONLY
mov [ofn.lpstrTitle],NULL
invoke  GetOpenFileName,ofn
    

;the following code is meant to show an open file dialog
format PE GUI 4.0
entry start

include '%include%/win32a.inc'

section '.data' data readable writeable

  hinstance dd ?

  ofn OPENFILENAME
  fname rb 100h

section '.code' code readable executable

  start:
        ;get the instance handle
        invoke  GetModuleHandle,0
        mov     [hinstance],eax

        ;init the OPENFILENAME structure
        mov     [ofn.lStructSize], sizeof.OPENFILENAME
        mov     eax,[hinstance]
        mov     [ofn.hInstance],eax
        mov     [ofn.hwndOwner],HWND_DESKTOP

        ;show the open dialog
        invoke  GetOpenFileName, ofn

        ;exit the program
        invoke  ExitProcess, hinstance

section '.idata' import data readable writeable

  library kernel,'KERNEL32.DLL',\
          comdlg,'COMDLG32.DLL'

  import kernel,\
         GetModuleHandle,'GetModuleHandleA',\
         ExitProcess,'ExitProcess'

  import comdlg,\
         GetOpenFileName,'GetOpenFileNameA'    

section '.data' data readable writeable
;...
fname dd ?
msg_title  db 'Path',0
;...
section '.code' code readable executable
  start:
        ;get the instance handle
        invoke  GetModuleHandle,0
        mov     [hinstance],eax

        ;init the OPENFILENAME structure
        mov     [ofn.lStructSize], sizeof.OPENFILENAME
        mov     eax,[hinstance]
        mov     [ofn.hInstance],eax
        mov     [ofn.hwndOwner],HWND_DESKTOP

        ;added to be able to get the file's path later
        mov     [ofn.lpstrFile],fname

        ;show the open dialog
        invoke  GetOpenFileName,ofn ;<-- it won't show! because of the fname var, i think

        ;test if I can get the file name
        invoke  MsgBox,HWND_DESKTOP,fname,msg_title,MB_OK

        ;exit the program
        invoke  ExitProcess, hinstance 
    

buffer rb 100h
;....
mov    [fname],buffer
mov    [ofn.lpstrFile],[fname]
;or directly
;mov    [ofn.lpstrFile],buffer
    

format PE GUI 4.0
entry start

include '%include%\win32a.inc'

section '.data' data readable writeable
  hinstance dd ?
  buffer rb 100h
  fname dd ?
  msg_title  db 'Path',0
  ofn OPENFILENAME

section '.code' code readable executable
  start:
        ;get the instance handle
        invoke  GetModuleHandle,0
        mov     [hinstance],eax

        ;init the OPENFILENAME structure
        mov     [ofn.lStructSize], sizeof.OPENFILENAME
        mov     eax,[hinstance]
        mov     [ofn.hInstance],eax
        mov     [ofn.hwndOwner],HWND_DESKTOP

        ;added to be able to get the file's path later
        mov     [fname],buffer
        mov     [ofn.lpstrFile],fname ;<-- If I comment this out, the next line works, but then I won't get the filename ;P
; and if i do: mov [ofn.lpstrFile],[fname] I get a compiler error, Invalid operand.

        ;show the open dialog
        invoke  GetOpenFileName,ofn ;<-- still won't show if I have something in the ofn.lpstrFile

        ;test if I can get the file name
        invoke  MsgBox,HWND_DESKTOP,fname,msg_title,MB_OK

        ;exit the program
        invoke  ExitProcess, hinstance

section '.idata' import data readable writeable
  library kernel,'KERNEL32.DLL',\
          comdlg,'COMDLG32.DLL',\
          user,'USER32.DLL'

  import kernel,\
         GetModuleHandle,'GetModuleHandleA',\
         ExitProcess,'ExitProcess'

  import comdlg,\
         GetOpenFileName,'GetOpenFileNameA'

  import user,\
         MsgBox,'MessageBoxA'                
    

        mov     [ofn.lStructSize], sizeof.OPENFILENAME 
        mov     eax,[hinstance] 
        mov     [ofn.hInstance],eax 
        mov     [ofn.hwndOwner],HWND_DESKTOP         
        mov     [ofn.nMaxFile],100h  ;this isn't requeried, but i think it schuld be here
        mov     [ofn.lpstrFile],buffer
        invoke  GetOpenFileName,ofn
        invoke  MsgBox,HWND_DESKTOP,buffer,msg_title,MB_OK
</code]
Now it works
    

HeapCreate + HeapAlloc?

Use strlen to get length of string.

  szString1 db 'This string',0
...

  ; create a new string
  stdcall StrNew, 0
  mov     [ebx+TComment.pContent],eax ; eax is the handle to the string
  ; copy the "standard byte string" in esi to it:
  mov     esi, szString1
  stdcall StrCopy, eax, esi      ; copy/set the filename

  stdcall StrLen, [ebx+TComment.pContent] ; returns length in eax
  stdcall StrLen, szString1 ; van be used on normal strings too.

  stdcall StrPtr, [ebx+TComment.pContent] ; get the pointer to the string, so we can for instance print it to stdout:
  stdcall PrintStringLn, eax ; (non StrLib function)
    

stdcall StrLen, my_string    

;--< How to use it >-----------------------------------------------------------------
; 1. Include "strutils.inc" somewhere in the begining of main file.
    

; 2. Define structure "StrTable" of type "TStrTable" somewhere in data section.
    

; 3. Before using of library functions, call "InitStrings"
    

; 4. After last use of the library (probably on close of application),
;    call "FreeStrings" to free all used memory.
; 5. Some functions are with register parameter passing, other with "stdcall"
;    parameter passing. Read the header descriptions of the functions.
;------------------------------------------------------------------------------------