QT COM Support For Dual Interface

hskoglund

Hi, if you've implemented your COM classes with a Dual interface, that is good news because then you use Qt's COM support in the class QAxObject.

First, add QT += axcontainer to your .pro file. Then add
#include <QAxObject> to your .cpp file.

After that you should be able to avoid doing CoCreateInstance() and instead lean on QAxObject. Here is the proverbial example (interacting with an Excel spreadsheet):

auto excel     = new QAxObject("Excel.Application");
auto workbooks = excel->querySubObject("Workbooks");
auto workbook  = workbooks->querySubObject("Add");
auto sheets    = workbook->querySubObject("Worksheets");
// count # of worksheets
int count = sheets->dynamicCall("Count()").toInt();

And you can still do a QueryInterface on the QAxObject say like this:

...
#include <quuid.h>
#include <atlbase.h> 

auto excel = new QAxObject("Excel.Application");
CComPtr<IDispatch> pDispatch;
excel->queryInterface(QUuid(IID_IDispatch),(void**) &pDispatch);
...

Madhura

@hskoglund
But our requirement is to use only cocreateinstance, Because our
Project is fully converted to visual studio solution.Is here any way to achieve same process using cocreateinstance ?

Our client application will be in native C++ basically we use only cocreateinstance for COM-Object

hskoglund

Hi, a big plus of using Active Qt is that don't have to any CoCreateInstance() yourself. Most of the COM documentation is written before Internet started (in the previous century) so good examples can be hard to find :-(

I found an old program of mine that queries the system monitor for uptime (it is written for Windows 2000 but should work equally well on Windows 10 or 11) and it does a CoCreateInstance:

// convert the ProgID of the DLL you want to load to a  CLSID
    CLSID clsid;
    HRESULT hr = CLSIDFromString(L"SysMon.3",&clsid);
    if (FAILED(hr))
        qFatal("CLSIDFromString() didn't work");

// try to load the DLL
    CComPtr<IUnknown> pUnknown;
    hr = CoCreateInstance(clsid,NULL,CLSCTX_INPROC_SERVER,IID_IUnknown,(LPVOID*) &pUnknown);
    if (FAILED(hr))
        qFatal("CoCreateInstance() didn't work");

with Qt it is slightly easier:

 auto qtComObject = new QAxObject("SysMon.3");

either way you load your .COM dll, now you can query into it either via Qt's QAxObject methods or the old way using QueryInterface. If found some old code that asks the .dll for its type info (note: this requires that the COM .dll has a dual interface implementation):

#include "mainwindow.h"
#include <QApplication>

#include "qaxobject.h"
#include "quuid.h"
#include "qdebug.h"
#include <atlbase.h>

void partyOnTypeLibrary(CComPtr<ITypeLib> pTypeLib)
{
// BSTR to QString converter
    auto bstr2QString = [](BSTR bstr) { return QString::fromUtf16((ushort*) bstr); };

// show type library's doc strings
    CComBSTR bstrName,bstrDocString;
    pTypeLib->GetDocumentation(-1,&bstrName,&bstrDocString,NULL,NULL);
    qDebug() << bstr2QString(bstrName) << bstr2QString(bstrDocString) << ":\n";

// step thru all the type info
    for (UINT u = 0; (u < pTypeLib->GetTypeInfoCount()); ++u)
    {
    // get the iTypeInfo ptr
        CComPtr<ITypeInfo> pTypeInfo;
        if FAILED(pTypeLib->GetTypeInfo(u,&pTypeInfo))
            continue;

    // get type flavor and name
        TYPEKIND typeKind;
        if FAILED(pTypeLib->GetTypeInfoType(u,&typeKind))
            continue;

        CComBSTR bstrName;
        if FAILED(pTypeInfo->GetDocumentation(-1,&bstrName,NULL,NULL,NULL))
            continue;
        QString sName = bstr2QString(bstrName);

    // get the type attribute
        TYPEATTR* pTypeAttr;
        if FAILED(pTypeInfo->GetTypeAttr(&pTypeAttr))
            continue;

    // check the type flavor, we support enums, interfaces and coclasses
        if (TKIND_ENUM == typeKind)
        {
            QString sEnums;
            for (int v = 0; (v < pTypeAttr->cVars); ++v)
            {
                VARDESC* pVarDesc;
                if FAILED(pTypeInfo->GetVarDesc(v,&pVarDesc))
                    break;

                if (v > 0)
                    sEnums += " ";
                if (VAR_CONST == pVarDesc->varkind)
                    sEnums += QString::number(pVarDesc->lpvarValue->lVal);

                pTypeInfo->ReleaseVarDesc(pVarDesc);
            }

            qDebug() << "Enum type:" << sName << sEnums;
        }

        if (TKIND_INTERFACE == typeKind)
            qDebug() << "Interface type:" << sName;

        if (TKIND_DISPATCH == typeKind)
            qDebug() << "IDispatch callable type:" << sName;

    // any function descriptors? they usually exist for interfaces and IDispatch
        for (int f = 0; (f < pTypeAttr->cFuncs); ++f)
        {
            FUNCDESC* pFuncDesc;
            if FAILED(pTypeInfo->GetFuncDesc(f,&pFuncDesc))
                break;

            if (pFuncDesc->wFuncFlags > 0)
                continue;   // skip these chaps (boring)

            if (pFuncDesc->invkind > INVOKE_PROPERTYGET)
                continue;   // skip put and putref properties

        // get the prop/func name and all the arg names
            QStringList slNames;
            BSTR aBS[1000];     // 1000 should suffice
            UINT uNames = 0;
            if FAILED(pTypeInfo->GetNames(pFuncDesc->memid,aBS,1000,&uNames))
                break;

            for (uint u = 0; (u < uNames); ++u)
                slNames.append(bstr2QString(aBS[u]));

            QString sKind = "Property";
            if (pFuncDesc->invkind == INVOKE_FUNC)
                sKind = "Function";
            QString sPropOrFuncName = slNames.first();
            slNames.removeFirst();

            qDebug() << sKind << sPropOrFuncName << slNames;
        }
        qDebug() << "";

    // we only care about coclasses at this point
        if (TKIND_COCLASS != typeKind)
            continue;

        qDebug() << "CoClass:" << bstr2QString(bstrName) << ":";

    // step through the implemented types for this coclass
        for (int i = 0; (i < pTypeAttr->cImplTypes); ++i)
        {
            HREFTYPE hRefType;
            if FAILED(pTypeInfo->GetRefTypeOfImplType(i,&hRefType))
                continue;

            CComPtr<ITypeInfo> pRefTypeInfo;
            if FAILED(pTypeInfo->GetRefTypeInfo(hRefType,&pRefTypeInfo))
                continue;

            CComBSTR bstrInterfaceName;
            if FAILED(pRefTypeInfo->GetDocumentation(-1,&bstrInterfaceName,NULL,NULL,NULL))
                continue;

            qDebug() << "        " << bstr2QString(bstrInterfaceName);
        }
    }
}

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
/*
// convert to CLSID
    CLSID clsid;
    HRESULT hr = CLSIDFromString(L"SysMon.3",&clsid);
    if (FAILED(hr))
        qFatal("CLSIDFromString() didn't work");

// trying to load the DLL
    CComPtr<IUnknown> pUnknown;
    hr = CoCreateInstance(clsid,NULL,CLSCTX_INPROC_SERVER,IID_IUnknown,(LPVOID*) &pUnknown);
    if (FAILED(hr))
        qFatal("CoCreateInstance() didn't work");
*/
.// here we use Qt's QAxObject
    auto qtComObject = new QAxObject("SysMon.3");

    CComPtr<IDispatch> pDispatch;
//    pUnknown->QueryInterface(QUuid(IID_IDispatch),(void**) &pDispatch);
    qtComObject->queryInterface(QUuid(IID_IDispatch),(void**) &pDispatch);
    if (nullptr == pDispatch)
        qFatal("Error: an IDispatch interface was not found.");

// get the ITypeInfo ptr
    UINT uTypeInfo;
    HRESULT hr = pDispatch->GetTypeInfoCount(&uTypeInfo);
    if (FAILED(hr) || (uTypeInfo < 1))
        qFatal("Sorry, could not locate any type information");
    if (1 != uTypeInfo)
        qFatal("Expected GetTypeInfoCount() to return 1");

    CComPtr<ITypeInfo> pTypeInfo;
    if FAILED(pDispatch->GetTypeInfo(0,LOCALE_SYSTEM_DEFAULT,&pTypeInfo))
        qFatal("Error: GetTypeInfo() failed");

// ok have ITypeInfo, use it to get the ITypeLib
    CComPtr<ITypeLib> pTypeLib;
    UINT uTypeInfoIndex;
    if FAILED(pTypeInfo->GetContainingTypeLib(&pTypeLib,&uTypeInfoIndex))
        qFatal("Error: GetContainingTypeLib() failed");

// party on this type library
   partyOnTypeLibrary(pTypeLib);
}

Madhura

Hello,
Thank for the support. We are using this sample and exposing the API inside C++ application.
Can you suggest the use query interface or "CoCreateInstance" inside C++ application.

Madhura

@hskoglund
We are having 2 different tlbs and in this case both are not able to communicate while using queryinterface it returns NULL

Kindly refer above sample as we provided.

hskoglund

Hi, I looked at the sample you mentioned "TN065: Dual-Interface Support for OLE Automation Servers"
However, this sample is from the previous century.
To get it running ok you need Microsoft Visual Studio 2008 or earlier, later versions of Visual Studio might not compile it correctly.
It is written 100% in MFC not Qt so how to make this sample work is not a question for this forum I think :-(

Madhura

@hskoglund

Is there any thing we need to add in (Project.exe) and another project (project.dll) to link each other and I can queryinterface from project.exe to get project.dll functions.

where project.exe and project.dll are different projects.

hskoglund : (note: this requires that the COM .dll has a dual interface implementation):

As you mentioned is there any sample we can check how to write a QT-COM in dual interface implementation.

hskoglund

If you use Qt to develop a COM app, then you always get a dual interface implementation for free (it's built-in).
There are some examples available, to get started check the COM App Example.
.

Madhura

@hskoglund

Yes we have created the QT-COM sample from QT documentation.from refrence link : ActiveQT COM app

We are able to connect main.exe and able to call its classes using tlb.

We have new tlb generated from different project and need to Queryinterface this new tlb (Call functions ) from our exe.

hskoglund

Please convert all your COM projects to Qt, then you will never have to do any calls to CoCreateInstance() and QueryInterface().

Madhura

@hskoglund

@hskoglund : Qt to develop a COM app, then you always get a dual interface implementation for free

As you mentioned dual interface is automatically implementes in QT.

Is there any mapping done between the classes, Is there any macros available or any other way we can do it ?

In Microsoft there are some macro which will help us to map the classes : "BEGIN_INTERFACE_MAP"

BEGIN_INTERFACE_MAP(CAutoClickDoc, CDocument)
INTERFACE_PART(CAutoClickDoc, DIID_IAClick, Dispatch)
INTERFACE_PART(CAutoClickDoc, IID_IDualAClick, DualAClick)
END_INTERFACE_MAP()

hskoglund

Hi, the most important macros are QAXFACTORY_BEGIN, QAXCLASS and QAXTYPE.
You can find them inside the COM App example.

Madhura

@hskoglund
In C++ COM we have "BEGIN_INTERFACE_MAP" below is one example:

BEGIN_INTERFACE_MAP(CAutoClickDoc, CDocument)
INTERFACE_PART(CAutoClickDoc, DIID_IAClick, Dispatch)
INTERFACE_PART(CAutoClickDoc, IID_IDualAClick, DualAClick)
END_INTERFACE_MAP()

Here CDocument and DualAClick both are different projects, not related to each other.

But using BEGIN_INTERFACE_MAP macro, the client project can access DualAClick by QueryInterface on CDocument.

For e.g.
CComPtr<CDocument> cDoc;
CoCreateInstance(cDoc);
DualAClick pDAC = cDoc.QueryInterface (IDualAClick);

We have used below macro which will provide all data in single idl and tlb not for separate idl and tlb.

QAXFACTORY_BEGIN("{UUID}", "{UUID}")
QAXCLASS(MainClass)
QAXTYPE(SubClass)
QAXFACTORY_END()

Is there any Macro similar to BEGIN_INTERFACE_MAP in QT.

hskoglund

Yes that is the QAXFACTORY_BEGIN() macro.