What is the correct, modern, c++ to call the Office automation COM APIs

Question

I haven't used c++ much in years, and have no experience with using COM from c++. I've written some code in c# using Office interop to work with Word, Excel, and PowerPoint, but am struggling to port that to c++. I've found Office Automation Using Visual C++, which isn't super helpful. It describes 3 ways to do it:

with the #import directive (which it calls buggy and discourages), with MFC, or with pure c++.

The code sample for pure c++ is extremely old-school, and looks to have been written at least 15 years ago. And instead of coding against a strongly-typed interface, it does everything with strings and variants.

I'm trying to do the best of all worlds, the clean code of MFC, but in a modern c++/winrt environment, with smart pointers, etc. In c# for example, I can do:

Windows.Win32.PInvoke.CLSIDFromProgIDEx("Word.Application", out Guid clsid);
Word.Application app = Windows.Win32.PInvoke.GetActiveObject(clsid, null, out object obj) as Word.Application;
app.Activate();

In c++ I have:

CLSID clsid;
check_hresult(CLSIDFromProgID(L"Word.Application", &clsid));

com_ptr<::IUnknown> unk;
check_hresult(GetActiveObject(clsid, NULL, unk.put()));

But then how do I actually convert it to the interface?

I would use the comutil smart pointers and interface-wrappings, assuming you can properly `#import` the msword type library (I believe it is called `msword.tlb`, and is usually in the `c:/program files/Microsoft Office/OfficeNN` folder, where `NN` denotes the office version. Caveat emptor: I've never done this with Office365, only standard office local-installs, so ymmv). Google how `#import` can be used for type library and COM DLL import wrapping. — WhozCraig, Apr 01 '23 at 00:52
Using COM Automation (IDispatch, VARIANT types, etc) is hard in C++ (and still is) because it was designed back then for high-level languages such as VB/VBA. #import is the way to go and works fine with a bit of tweaking (https://stackoverflow.com/questions/67891994/error-while-creating-tlh-and-tli-file-using-import-from-msoutl-olb-in-vc-pr) but it's just a tool that create wrappers, it will still be difficult. IMHO C++/WinRT won't help you much here. Mixing language (like interop with C# which was designed to use COM Automation quite easily) is something to consider as it can be much easier. — Simon Mourier, Apr 01 '23 at 06:22

score 0 · Accepted Answer · answered Apr 18 '23 at 17:55

Thanks for the comments! I was able to get it working, for Word, and also Excel and PowerPoint, using #import. Despite that old documentation saying otherwise, I've not found it to be buggy, though certainly much harder to consume than from c#. Here's a code sample showing calling each of the different interfaces:

#import "C:\Program Files\Microsoft Office\Root\VFS\ProgramFilesCommonX64\Microsoft Shared\OFFICE16\MSO.dll"
using namespace Office;
#import "C:\Program Files\Microsoft Office\root\Office16\MSWORD.OLB" raw_native_types auto_rename
#import "C:\Program Files\Microsoft Office\root\Office16\MSPPT.OLB" raw_native_types
#import "C:\Program Files\Microsoft Office\root\Office16\EXCEL.EXE" raw_native_types auto_rename

using namespace winrt;
using namespace std;
using namespace wil;
using namespace Windows::Foundation;

template <typename T>
impl::com_ref<T> GetApplication(LPCOLESTR programId)
{
    CLSID clsid{};
    check_hresult(CLSIDFromProgID(programId, &clsid));

    com_ptr<::IUnknown> unk;
    impl::com_ref<T> app = SUCCEEDED(GetActiveObject(clsid, NULL, unk.put()))
        ? unk.as<T>()
        : create_instance<T>(clsid, CLSCTX_LOCAL_SERVER);

    return app;
}

int main()
{
    init_apartment();

    {
        auto app = GetApplication<PowerPoint::_Application>(L"PowerPoint.Application");
        unique_bstr path{};
        if (app->ActivePresentation)
        {
            path.reset(app->ActivePresentation->FullName);
            wcout << path.get() << endl;
        }
    }

    {
        auto app = GetApplication<Excel::_Application>(L"Excel.Application");
        wcout << app->ActiveWorkbook->GetFullName(0) << endl;
    }

    {
        auto app = GetApplication<Word::_Application>(L"Word.Application");
        wcout << app->ActiveDocument->FullName << endl;
    }
}

Unfortunately, you have to use variants for various operations with the API, here's an example of how I did that:

    unique_bstr path{};
    unique_variant page{};
    if (app->ActiveDocument)
    {
        path.reset(app->ActiveDocument->FullName);
        wcout << path.get() << endl;
        page.reset(app->Selection->Information[Word::WdInformation::wdActiveEndPageNumber]);
    }

    if (path)
    {
        unique_variant pathVariant{};
        InitVariantFromString(path.get(), &pathVariant);
        app->Documents->Open(&pathVariant);
        app->Activate();
        
        unique_variant goToPageVariant{};
        if (SUCCEEDED(InitVariantFromInt32(Word::WdGoToItem::wdGoToPage, &goToPageVariant)))
        {
            unique_variant nullVariant{};
            app->ActiveWindow->Selection->GoTo(&goToPageVariant, &nullVariant, &page, &nullVariant);
        }
    }

What is the correct, modern, c++ to call the Office automation COM APIs

1 Answers1