How to open existing file using COM/ATL (no MFC)

#include <windows.h>
#include <iostream>

using namespace std;

#import "C:\Program Files\Common Files\Microsoft Shared\OFFICE11\MSO.DLL"
    rename("RGB", "MSORGB")
    rename("DocumentProperties", "MSDocumentProperties")

using namespace Office;

#import "C:\Program Files\Common Files\Microsoft Shared\VBA\VBA6\VBE6EXT.OLB"

using namespace VBIDE;

#import "C:\Program Files\Microsoft Office\OFFICE11\EXCEL.EXE"
    rename("DialogBox", "ExcelDialogBox" )
    rename("RGB", "ExcelRGB")
    rename("CopyFile", "ExcelCopyFile")
    rename("ReplaceText", "ExcelReplaceText")

void DumpCOMError(_com_error& e) {
    wcout << L"Error:" << endl;
    wcout << L"  Code = " << hex << e.Error() << endl;
    wcout << L"  Code meaning = " << e.ErrorMessage() << endl;
    _bstr_t bstrSource(e.Source());
    _bstr_t bstrDescription(e.Description());
    wcout << L"  Source = " << bstrSource << endl;
    wcout << L"  Description = " << bstrDescription << endl;
}

HRESULT IsXlsFileOpen(LPWSTR FileName, BOOL& file_open) {
    Excel::_ApplicationPtr pApplication;
    HRESULT hr = E_FAIL;
    if (FAILED(hr = pApplication.CreateInstance(L"Excel.Application"))) {
        file_open = FALSE;
        return hr;
    }

    _variant_t  varOption(static_cast<long>(DISP_E_PARAMNOTFOUND), VT_ERROR);
    Excel::_WorkbookPtr pBook;

    try {
        pBook = pApplication->Workbooks->Open(
                        FileName,
                        varOption,
                        varOption,
                        varOption,
                        varOption,
                        varOption,
                        varOption,
                        varOption,
                        varOption,
                        varOption,
                        varOption,
                        varOption,
                        varOption);

        file_open = pBook->ReadOnly == VARIANT_TRUE;
        pBook->Close(VARIANT_FALSE);

        hr = S_OK;
    } catch (_com_error& e) {
        file_open = FALSE;
        DumpCOMError(e);
        hr = e.Error();
    }

    pApplication->Quit();
    return hr;
}

int main(int argc, wchar_t* argv[])
{
    CoInitialize(NULL);
    {
        BOOL fileOpen;
        HRESULT hr = IsXlsFileOpen(L"f:\temp\treta.xls", fileOpen);
        if (SUCCEEDED(hr)) {
            cout << "File is " << (fileOpen ? "open" : "not open") << "." << endl;
        }
        cout << "IsXlsFileOpen returned: 0x" << hex << hr << endl;
    }
    CoUninitialize();

    return 0;
}