Using the same hWnd for MSG_ENABLEDS doesn't work

[Martin Walter Tan]

I was able to get through States 1-5 thanks to the TWAIN examples and posts I read in this forum. I was able to open the DSM using my application’s window handle:

 
  TW_UINT16 rc;
  // open source manager
  rc = (*DSM_Entry)(&AppID, NULL, DG_CONTROL, DAT_PARENT, MSG_OPENDSM, (TW_MEMREF) hWnd);
  std::cout << "Open Source Manager: " << rc << std::endl;
  // open select source dialog
  rc = (*DSM_Entry)(&AppID, NULL, DG_CONTROL, DAT_IDENTITY, MSG_USERSELECT, (TW_MEMREF) &SourceID);
  std::cout << "Open Select Source Dialog: " << rc << std::endl;

I get the hWnd from a char * casted to (HWND) which is passed from my original application which is not native C++.
User select source dialog works. But then when I try to send MSG_ENABLEDS it doesn’t work and fails with return code (rc) TWRC_FAILURE:

  TW_USERINTERFACE ui;
  ui.ShowUI = (TW_BOOL) TRUE;
  ui.ModalUI = (TW_BOOL) TRUE;
  ui.hParent = hWnd;
  rc = (*DSM_Entry)(&AppID, &SourceID, DG_CONTROL, DAT_USERINTERFACE, MSG_ENABLEDS, (TW_MEMREF) &ui);
  std::cout << "Enable DS (Open Dialog): " << rc << std::endl;

However, I was able to make it work when I found another guy’s example, he made a new window using CreateWindow:

  // https://www.twain.org/forums/topic/direct-acquire-without-ui
  HWND test = CreateWindow("STATIC",  "", WS_POPUPWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, HWND_DESKTOP, 0, hModule, 0);

  TW_USERINTERFACE ui;
  ui.ShowUI = (TW_BOOL) TRUE;
  ui.ModalUI = (TW_BOOL) TRUE;
  ui.hParent = test;
  rc = (*DSM_Entry)(&AppID, &SourceID, DG_CONTROL, DAT_USERINTERFACE, MSG_ENABLEDS, (TW_MEMREF) &ui);
  std::cout << "Enable DS (Open Dialog): " << rc << std::endl;

I would like to ask why it doesn’t work when I use my original window handle (hWnd) and it works when I spawn a new window using win32 api?

[markm]

The most likely candidate is a threading issue. All operations on a window must occur within the same thread where the window was created.
MSG_ENABLEDS must occur in the same thread where CreateWindow was called. Adding the extra CreateWindow probably took care of that.

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