How to execute code after the dialog is just shown to the user in Windows MFC.

DialogShown.cpp

// Definition of the message code
#define WM_DIALOG_SHOW              (WM_APP + 10)


// Functions that are members of the dialog class
class CMyDlg : public CDialog
{
    ...
    afx_msg void OnWindowPosChanged( WINDOWPOS* lpwndpos );
    afx_msg void OnDialogShow(WPARAM wp, LPARAM lp);
    ...
} ;
    

// Declaration of message map so that the functions are called    
BEGIN_MESSAGE_MAP(CMyDlg, CDialog)
    ...
    ON_WM_WINDOWPOSCHANGED()
    ON_MESSAGE(WM_DIALOG_SHOW, OnDialogShow )
    ...
END_MESSAGE_MAP()


// Functions called when the messages are received by the dialog
// If you only want to execute the code once, you might have to use some sort of flag
void CMyDlg::OnWindowPosChanged( WINDOWPOS* lpwndpos )
{
  CWnd::OnWindowPosChanged( lpwndpos );

    if( lpwndpos->flags & SWP_SHOWWINDOW )
    {
        PostMessage( WM_DIALOG_SHOW, 0, 0);
    }
}

void CMyDlg::OnDialogShow(WPARAM wp, LPARAM lp)
{
    MessageBox( "Here we are", "Title" ) ;
}