How do I call functions that receive floating point values using IDispatch.Invoke?

c++, winapi, windows

Solution

Well, that was a head-scratcher. The documentation for this "method" is very misleading, it is not a method. It behaves more like an indexed property and requires the DISPATCH_METHOD | DISPATCH_PROPERTYGET flags. CComPtr doesn't support that, you'll need to spin this by hand. This code worked:

    CComVariant result;
    CComVariant centimeters((float)2.0);
    DISPID dispid;
    LPOLESTR name = L"CentimetersToPoints";
    HRESULT hr = wordapp->GetIDsOfNames(IID_NULL, &name, 1, LOCALE_USER_DEFAULT, &dispid);
    assert(SUCCEEDED(hr));

    DISPPARAMS dispparams = { &centimeters, NULL, 1, 0};
    hr = wordapp->Invoke(dispid, IID_NULL, LOCALE_USER_DEFAULT,
            DISPATCH_METHOD | DISPATCH_PROPERTYGET, &dispparams, &result, nullptr, nullptr);
    assert(SUCCEEDED(hr));

Problem

I'm having trouble calling a function that has a floating point argument and a floating point result, using `IDispatch.Invoke`. Here's a minimal reproduction: ``` #include <atlbase.h> #include <comutil.h> int main(int argc, char* argv[]) { CoInitialize(NULL); CComPtr<IDispatch> wordapp; if (SUCCEEDED(wordapp.CoCreateInstance(L"Word.Application", NULL, CLSCTX_LOCAL_SERVER))) { CComVariant result; CComVariant centimeters((float)2.0); CComVariant retval = wordapp.Invoke1(L"CentimetersToPoints", &centimeters, &result); } return 0; } ``` I'm using the ATL `CComPtr` to make things cleaner. But it's a very loose wrapper around `IDispatch.Invoke`. When I run this, the call to `Invoke1` fails and returns `E_FAIL`. I suspect that the problem is related to the use of floating point arguments and/or return value. If I call a function that does not use such values, the invoke succeeds: ``` CComVariant retval = wordapp.Invoke0(L"ProductCode", &result); ``` I note that if I call the function from VBS, or from PowerShell, it succeeds. I'm presuming that they both use late bound `IDispatch` and so that would indicate that what I am attempting is at least possible. So, how can I call this function using `IDispatch`?

Original source