HWND hwndTarget = FindWindow(NULL,"My VB6 Form");
  if (hwndTarget!=0){
    std::string buffer = "My string - Hello VB6";
    COPYDATASTRUCT cds;
    cds.dwData=1;
    cds.cbData = static_cast<DWORD>(buffer.length()) + 1;
    cds.lpData = const_cast<char *>((buffer.c_str()));
    SendMessage(hwndTarget, WM_COPYDATA, 0, (LPARAM) &cds);
  }

SendMessage (hwndС-шноеПриложение, WM_COPYDATA, 0, cds)

PCREATE_PROCESS(CProga);
...
void CProga::Main()
...
  for (;;) {
    PThread::Sleep(500);
    ...
    if( (_access( "job.txt", 0 )) != -1 ){
       //читать файл
       //выполнять какой-то код
    }

  }

void CProga::Main()
{
...
  if (!Initialise()) {
    PThread::Sleep(100);  // workaround for race condition
    return;
  }
  char t[500];
  GetConsoleTitleA( t, 500 );     // retrieve the text of the title bar of the console window
  HWND hwndConsole = FindWindowA( NULL, t );
  HINSTANCE hInstance = (HINSTANCE)(LONG_PTR)GetWindowLong(hwndConsole, GWL_HINSTANCE);
  static const char* class_name = "DUMMY_CLASS";
  WNDCLASSEX wx = {};
  wx.cbSize = sizeof(WNDCLASSEX);
  wx.lpfnWndProc = WndProc;        // function which will handle messages
  wx.hInstance = hInstance;
  wx.lpszClassName = class_name;
  if ( RegisterClassEx(&wx) ) {
    HWND hwndWindow = CreateWindowEx( 0, class_name, "dummy_name", 0, 0, 0, 0, 0, HWND_MESSAGE, NULL, NULL, NULL );
    cout << "Window Created" << "\n";
    // Enter regular message loop, to process messages for
    // our window.
    MSG msg;
    while( GetMessage( &msg, hwndWindow, 0, 0 ) )
    {
      TranslateMessage(&msg);
      DispatchMessage(&msg);
    }
  }
...
  for (;;) {
    PThread::Sleep(500);
    ...
    if( (_access( "job.txt", 0 )) != -1 ){
       //читать файл
       //выполнять какой-то код
    }

}

LRESULT CALLBACK WndProc( HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam )
{
  switch( message )
  {
    case WM_COPYDATA:
      cout << "WM_COPYDATA" << endl;
      COPYDATASTRUCT* pcds = (COPYDATASTRUCT*)lparam;
      if (pcds->dwData == 1)
      {
        LPCTSTR lpszString = (LPCTSTR)(pcds->lpData);
        cout << lpszString << "(" << pcds->cbData << ")\n";
      }
      break;
  }
  return DefWindowProc( hwnd, message, wparam, lparam );
}

void CProga::Main()
{
...
  if (!Initialise()) {
    PThread::Sleep(100);  // workaround for race condition
    return;
  }
  for (;;)
    PThread::Sleep(5000);
}

void CProga::Main()
...
  for (;;) {
    PThread::Sleep(500);
    ...
    if( (_access( "job.txt", 0 )) != -1 ){
       //читать файл
       //выполнять какой-то код
    }
    if( (_access( "otherfile.txt", 0 )) != -1 ){
       //делай другие действия

  }

void CProga::Main()
...
    MSG msg;
    while( GetMessage( &msg, hwndWindow, 0, 0 ) )
    {
      TranslateMessage(&msg);
      DispatchMessage(&msg);
    }

 for (;;) {
    PThread::Sleep(500);
    ...
    if( (_access( "job.txt", 0 )) != -1 ){
       //читать файл
       //выполнять какой-то код
    }
    if( (_access( "otherfile.txt", 0 )) != -1 ){
       //делай другие действия

  }

void CProga::Main()
...
    MSG msg;
    while( GetMessage( &msg, hwndWindow, 0, 0 ) )
    {
      TranslateMessage(&msg);
      DispatchMessage(&msg);
    }

 for (;;) {
    PThread::Sleep(500);
    ...
    if( (_access( "job.txt", 0 )) != -1 ){
       //читать файл
       //выполнять какой-то код
    }
    if( (_access( "otherfile.txt", 0 )) != -1 ){
       //делай другие действия

  }

void CProga::Main()
{
...
  if (!Initialise()) {
    PThread::Sleep(100);  // workaround for race condition
    return;
  }

  DWORD dwThreadId, dwThrdParam = 1;
  HANDLE hThread;
  hThread = CreateThread(
   NULL,         // атрибуты безопасности по умолчанию
   0,            // размер стека используется по умолчанию
   ThreadFunc,   // функция потока
   &dwThrdParam, // аргумент функции потока
   0,            // флажки создания используются по умолчанию
   &dwThreadId); // возвращает идентификатор потока

  if (hThread == NULL)
  {
    cout << "CreateThread failed.\n"; 
  }
  else {
    cout << "CreateThread OK.\n"; 
    CloseHandle( hThread );
  }

  for (;;) {
    PThread::Sleep(500);
    ...
    if( (_access( "job.txt", 0 )) != -1 ){
       //читать файл
       //выполнять какой-то код
    }

}

LRESULT CALLBACK WndProc( HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam )
{
  switch( message )
  {
    case WM_COPYDATA:
      cout << "WM_COPYDATA" << endl;
      COPYDATASTRUCT* pcds = (COPYDATASTRUCT*)lparam;
      if (pcds->dwData == 1)
      {
        LPCTSTR lpszString = (LPCTSTR)(pcds->lpData);
        cout << lpszString << "(" << pcds->cbData << ")\n";
      }
      break;
  }
  return DefWindowProc( hwnd, message, wparam, lparam );
}

DWORD WINAPI ThreadFunc(LPVOID /*lpParam*/)
{
  char t[500];
  GetConsoleTitleA( t, 500 );     // retrieve the text of the title bar of the console window
  HWND hwndConsole = FindWindowA( NULL, t );
  HINSTANCE hInstance = (HINSTANCE)(LONG_PTR)GetWindowLong(hwndConsole, GWL_HINSTANCE);
  static const char* class_name = "DUMMY_CLASS";
  WNDCLASSEX wx = {};
  wx.cbSize = sizeof(WNDCLASSEX);
  wx.lpfnWndProc = WndProc;        // function which will handle messages
  wx.hInstance = hInstance;
  wx.lpszClassName = class_name;
  if ( RegisterClassEx(&wx) ) {
    HWND hwndWindow = CreateWindowEx( 0, class_name, "dummy_name", 0, 0, 0, 0, 0, HWND_MESSAGE, NULL, NULL, NULL );
    cout << "Window Created" << "\n";
    // Enter regular message loop, to process messages for
    // our window.
    MSG msg;
    while( GetMessage( &msg, hwndWindow, 0, 0 ) )
    {
      TranslateMessage(&msg);
      DispatchMessage(&msg);
    }
  }
  return 0;
}

LRESULT CALLBACK WndProc( HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam )
{
  switch( message )
  {
    case WM_COPYDATA:
...
      return 0;
      break;
    case WM_TIMER:
...
      return 0;
      break;
  }
  return DefWindowProc( hwnd, message, wparam, lparam );
}

  switch( message )
  {
    case WM_COPYDATA:
    case WM_TIMER:
      //единый код для WM_COPYDATA и WM_TIMER
      break;
    case WM_...
  }

Public Function NewFormWindowProc _
 (ByVal hwnd As Long, ByVal Msg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
 
  Select Case Msg
    Case WM_COPYDATA
       'код обработки WM_COPYDATA
    Сase WM_TIMER
       'код обработки WM_TIMER
    Case Else
  End Select
  
  NewFormWindowProc = CallWindowProc _
   (OldFormWindowProc, hwnd, Msg, wParam, lParam)
End Function

Public Function NewFormWindowProc _
 (ByVal hwnd As Long, ByVal Msg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
 
  Select Case Msg
    Case WM_COPYDATA
       'код обработки WM_COPYDATA
       NewFormWindowProc = 0
       Exit Function
    Сase WM_TIMER
       'код обработки WM_TIMER
       NewFormWindowProc = 0
       Exit Function
    Case Else
  End Select
  
  NewFormWindowProc = CallWindowProc _
   (OldFormWindowProc, hwnd, Msg, wParam, lParam)
End Function

      return 0;
      break;

LRESULT CALLBACK WndProc( HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam )
{
  switch( message )
  {
    case WM_COPYDATA:
...
      break;
    case WM_TIMER:
...
      break;
  }
  return DefWindowProc( hwnd, message, wparam, lparam );
}

    Сase...
...
       Exit Function