Selectable file formats at file open/save in MFC in Visual C++ - mfc

Question

My English is not perfect, sorry.
I am using Visual C++ 2019 Community, with MFC.
At CFileDialog class, I wish choose the file encoding: UTF-16 (little/big endian), UTF-8, ANSI, etc, at saving, with or without signature (2 or bytes what signs the encoding, at the begin of the file). This should be contacted to open/save button. In documentation of CFileDialog, I can add only separate buttons, not extending the open/save button like in Visual Studio, LibreOffice, etc. How can I do this? I am beginner with MFC, and desktop programs, but not beginner with C++. Thank you.

Answer 1

In the comments to your question you state:
Yes, I know. But there is no place to write this. Handler of Open menu is built-in part of MFC. At the source code, CFileDialog is not happen. I added handler OnFileOpen(). This has not paramters, empty at now, and the open menu item manages well. So, I do not know to where I type OFN_ALLOWMULTISELCT.
If you look at Technical Note 22 it mentions:
ID_FILE_OPEN Opens an existing document.
Note
You must connect this to your CWinApp-derived class's message map to
enable this functionality.
CWinApp::OnFileOpen has a very simple implementation of calling
CWinApp::DoPromptFileName followed by CWinApp::OpenDocumentFile with
the file or path name of the file to open. The CWinApp implementation
routine DoPromptFileName brings up the standard FileOpen dialog and
fills it with the file extensions obtained from the current document
templates.
One common customization of ID_FILE_OPEN is to customize the FileOpen
dialog or add additional file filters. The recommended way to
customize this is to replace the default implementation with your own
FileOpen dialog, and call CWinApp::OpenDocumentFile with the
document's file or path name. There is no need to call the base class.
As you can see, it states:
The CWinApp implementation
routine DoPromptFileName brings up the standard FileOpen dialog and
fills it with the file extensions obtained from the current document
templates.
But DoPromptFileName seems to be an undocumented function. You can either:
Debug into to MFC source code to see what it does and override it in your own app class,
Continue to roll out your own CWinApp::OnFileOpen override which uses your own CFileDialog.
I suggest you also read up on CFileDialog constructor documentation because it will assist you for basic customization. However, it sounds to me you need to do what #sergiol said in the comments and display your own CFileDialog (using either approach previously described) and add your own combo with your encoding options. Then handle accordingly.
Please note that I have no experience with that level of customization but it should get you going.