CreateComObject.hpp

#pragma once
#include <Windows.h>
#include <atlbase.h>


/** Programmatically creates an instance of a a COM object found in a specific DLL.
    Confirmed to work with UWP if the DLL is placed in the same "AppX" folder as the UWP app.

    WARNING: Will _leak_ handle to the DLL to avoid premature DLL unloading. The good
    way to handle this would be to also return a RAII wrapper of the DLL handle, and
    let the caller be responsible for freeing it after the object have been deleted. */
static CComPtr<IUnknown> CreateComObject (const std::wstring& fileName, const CLSID& classId) {
    // load DLL (handle is leaked)
    HMODULE dll_handle = LoadPackagedLibrary(fileName.c_str(), NULL);
    if (!dll_handle)
        return CComPtr<IUnknown>();

    // get class object
    typedef HRESULT(STDAPICALLTYPE *DllGetClassObject_t)(const CLSID& clsid, const IID& iid, void** ppv);
    DllGetClassObject_t dllGetClassObject = reinterpret_cast<DllGetClassObject_t>(GetProcAddress(dll_handle, "DllGetClassObject"));
    if (!dllGetClassObject)
        return CComPtr<IUnknown>();

    // get factory function
    CComPtr<IClassFactory> iClassFactory;
    HRESULT res = dllGetClassObject(classId, IID_IClassFactory, reinterpret_cast<void**>(&iClassFactory));
    if (FAILED(res))
        return CComPtr<IUnknown>();

    // create loader object
    CComPtr<IUnknown> instance;
    res = iClassFactory->CreateInstance(nullptr, IID_IUnknown, reinterpret_cast<void**>(&instance));
    if (FAILED(res))
        return CComPtr<IUnknown>();

    return instance;
}