wchar_t* FileNameBuffer = new wchar_t[1024*20];
CFileDialog Dlg(true, nullptr, nullptr, OFN_ALLOWMULTISELECT);
Dlg.m_ofn.lpstrFile = FileNameBuffer;
Dlg.m_ofn.nMaxFile = (1024 * 20) - 1;
INT_PTR a = Dlg.DoModal();
delete [] FileNameBuffer;
버퍼를 충분히 늘려주지 않으면 안됨(기본값 255)