dll and subclassing

[Chris Holbrook]

It's still early days for me with dlls.

My objective is to create a window in the exe and create then subclass a child window on it in the dll. I have established that DDT cannot be used in this way, not for the subclassed control anyway.

Whenever control is passed to the subclass proc, things start to go wrong, usually ending in a GPF. I've gone around it a couple of times without success - any suggestions?

the dll code

' dll and subclass problem
#compile dll
#dim all
#include "commctrl.inc"

global hCtlStatic as dword
global hDlg as dword
global OldProc as dword
'------------------------------------------------------------------------------
function SCProc ( hWnd as dword, byval wMsg as dword, byval wparam as dword, byval lparam as dword) export as long
    local s as string
    ? "SubClassProc"
    s = "replacement title" 'hex$(wmsg)
    sendmessage hDlg, %WM_SETTEXT, 0, strptr(s)

    function = callWindowProc(OldProc, hWnd, Wmsg, wParam, lParam)
    'function = defWindowProc(hWnd, Wmsg, wParam, lParam)

end function
'----------------------------------------------------------------------------
function CrWnd alias "CrWnd" ( hd as dword, lCtlStatic as long) export as dword
    local hinst, dw as dword
    local l as long

    initcommoncontrols
    hCtlStatic = CreateWindowEx(0,"Static","", _
              %ws_child or %ws_visible or %ws_tabstop or %ws_border, _
              205, 5, 200, 150, _
              hd, lCtlStatic, getmodulehandle(byval 0), byval %NULL)
    hDlg = hD
    ? "hWnd=" + str$(hCtlStatic) + str$(hd)
    ? "pre subclass"
    OldProc = SetWindowLong(hCtlStatic, %GWL_WNDPROC, codeptr(SCProc))
    ? "post subclass"
end function

the exe code

' DDT DLL problem
' Chris Holbrook 22-DEC-2008

#compile exe
#dim all
#include "WIN32API.INC"
%IDD_DIALOG1 = 101
%IDC_Static      = 1002

declare function CrWnd lib "dllscproblem.dll" alias "CrWnd" ( hd as dword, lctlStatic as long) as dword
'declare function SCProc lib "dllscproblem.dll" alias "SCProc" ( hWnd as dword, byval wMsg as dword, byval wparam as dword, byval lparam as dword) as long

'-----------------------------------------------------------------------
function My_MainWndProc _
  ( _
  byval hWnd    as dword, _ ' window handle
  byval uMsg    as dword, _ ' type of message
  byval wParam  as dword, _ ' first message parameter
  byval lParam  as long _   ' second message parameter
  ) export as long

    select case uMsg
        case %wm_destroy
            PostQuitMessage 0
            exit function
        case %wm_create
            CrWnd( hWnd, %IDC_Static)' Create a STATIC control
            function = 0
        exit function
    end select
    function = defWindowProc(hWnd, uMsg, wParam, lParam)
end function
'-----------------------------------------------------------------------
function winmain _
    ( byval hInstance     as long, _        ' handle of current instance
      byval hPrevInstance as long, _        ' handle of previous instance(not used in Win32)
      byval pszCmdLine    as asciiz ptr, _  ' address of command line
      byval nCmdShow      as long _         ' show state of window
) as long
    local szClassName   as asciiz * 32       ' class name
    local tWCX          as WNDCLASSEX               ' class information
    local tmsg          as tagMsg                   ' message information
    local hWnd, i, lresult          as dword

    ' Register the Form1 window
    szClassName        = "MY_CLASS"
    tWCX.cbSize        = sizeof(tWCX)                               ' size of WNDCLASSEX structure
    tWCX.style         = %CS_DBLCLKS  or %CS_HREDRAW or %CS_VREDRAW     ' class styles
    tWCX.lpfnWndProc   = codeptr(My_MainWndProc)                        ' address of window procedure used by class
    tWCX.cbClsExtra    = 0                                          ' extra class bytes
    tWCX.cbWndExtra    = 0                                          ' extra window bytes
    tWCX.hInstance     = hInstance                                  ' instance of the EXE/DLL that is registering the window
    tWCX.hIcon         = LoadIcon(%NULL, byval %IDI_APPLICATION)    ' handle of class icon
    tWCX.hCursor       = LoadCursor(%NULL, byval %IDC_ARROW)        ' handle of class cursor
    tWCX.hbrBackground = getstockobject(%white_brush)               ' brush used to fill background of window's client area
    tWCX.lpszMenuName  = %NULL                                      ' resource identifier of the class menu
    tWCX.lpszClassName = varptr(szClassName)                        ' class name
    tWCX.hIconSm       = LoadIcon(%NULL, byval %IDI_APPLICATION)    ' handle of small icon shown in caption/system Taskbar
    if isfalse RegisterClassEx(tWCX) then
      function = %FALSE
      exit function
    end if
    ' Create main window
    hWnd = CreateWindowEx(%ws_ex_windowedge, _
                          szClassName, _
                          "One day...",_
                          %ws_overlapped or %ws_visible or %ws_sysmenu, _
                           100, 100, 200, 100, _
                          %NULL, %NULL, hInstance, byval %NULL)
    ' fail if window is not created
    if isfalse hWnd then
      function = %FALSE
      exit function
    end if
    ' Activate window
    ShowWindow hWnd, %sw_show
    UpdateWindow hWnd
    while istrue GetMessage(tmsg, byval %NULL, 0, 0)
        if isfalse IsDialogMessage(hWnd, tmsg) then
            TranslateMessage tmsg
            DispatchMessage tmsg
        end if
    wend
end function

'-------------------------------------------------------------------------

[Chris Boss]

I am not sure what you are trying to accomplish, but at first glance you have a problem in your subclassing routine.

Be very careful of executing code outside of a Select case structure for the MSG parameter. For example you call:

sendmessage hDlg, %WM_SETTEXT, 0, strptr(s)

and it is not within a standard message processing select case structure.

ie.

SELECT CASE Msg
   CASE %WM_LBUTTONUP   ' some message to be processed
   CASE ELSE
END SELECT

in the subclass routine and it is not in a standard Select case structure for messages. Actually you don't have such a structure.

What happens is that the code that is not in such a structure, gets executed for "every single message" the window processes and there can be many. Especially when the mouse moves over a control, there can be hundreds of messages generated within a few seconds.

By calling sendmessage the way you do to set text for the parent window, this could happen many, many times very quickly.

While the API docs don't discuss this as far as I know, this can create what I would call an "overload" for Windows.

I don't know if this is the cause of your problem, but it surely is not a good practice.

[Chris Holbrook]

Thanks Chris - yes, I debugged the select statement out of existence. Replacing it with the code below doesn't change the outward signs much.

    select case wMsg
        case %wm_lbuttonup   ' some message to be processed
            s = "replace title"
            sendmessage hDlg, %WM_SETTEXT, 0, strptr(s)
        case else
    end select

[Chris Boss]

Found the problem!

Change:

hWnd as dword

to

BYVAL hWnd as dword

in the subclass procedures parameter list.

Forgetting the BYVAL in any of the parameters will definitely cause a crash.

[Chris Holbrook]

Chris, thanks a thousand!

[Cliff Nichols]

AMAZING how just 1 lil word can speak TONS!!!!!

Especially "BYVAL" and "BYREF" and how it is used in a language, or if omitted (what is substituted in its place)

Sometimes this is the cause in the ole

1. Works great
2. Suddenly fails
3. WHAT???? just happened????
4. Somethings wrong.....what changed?
5. Nothing changed (that I can find)....sooooo "WHATTTT CHANGED!!!!"
6. Nothing
7. Nothing
8. Nothing
9. Oppps missed a word, and a "Bug" has been there all the while and just now crept up