.Net COM and ActiveQt: return class instance doesn't work

Gundz

Hello,

For a project i'm getting familliar with .net COM and ActiveQt.
I'm able to get result from functions that are returning standard types (string, int...)

But returning a new instance of a class doesn't work:

my .net file .cs (.Net framework 4.8)


using System.Runtime.InteropServices;

namespace COMTest
{
    [ComVisible( true )]
    [Guid( "BAD219A5-CE0A-40EB-BC66-9C6D3B71F084" )]
    [ClassInterface( ClassInterfaceType.AutoDual )]
    public class COMTestSubclass
    {
        public string AGetString()
        {
            return "COMTestSubclass::AGetString()";
        }
    }

    [ComVisible( true )]
    [Guid( "05E563DB-EAA8-4C45-96DD-9A7B83CA1F63" )]
    [ClassInterface( ClassInterfaceType.AutoDual )]
    public class COMTestMain
    {
        public COMTestSubclass AGetSubclass()
        {
            return new COMTestSubclass();
	}

        public string AGetString()
        {
            return "COMTestMain::AGetString()";
        }
    }
}

my main.cpp file, comtest.h was generated by dumpcpp and the tlb file (Qt 6.3.0)

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

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);


    qInfo() << "Main:";
    COMTest::COMTestMain *main = new COMTest::COMTestMain();
    qInfo() << main->AGetString();
    qInfo() << main->ToString();
    qInfo() << "\n";

    qInfo() << "SubClass:";
    COMTest::COMTestSubclass *subclass = new COMTest::COMTestSubclass();
    qInfo() << subclass->AGetString();
    qInfo() << subclass->ToString();
    qInfo() << "\n";

    qInfo() << "SubClassPtr:";
    auto subclassPtr = main->AGetSubclass();
    qInfo() << subclassPtr->AGetString();
    qInfo() << subclassPtr->ToString();

    return 0;
}

16:55:07: Starting C:\Users\ME\Desktop\MixedInKeyTools\COMTest\build-QtCOMTest-Desktop_Qt_6_3_0_MinGW_64_bit-Debug\debug\QtCOMTest.exe...
Main:
"COMTestMain::AGetString()"
"COMTest.COMTestMain"


SubClass:
"COMTestSubclass::AGetString()"
"COMTest.COMTestSubclass"


SubClassPtr:
""
16:55:08: C:\Users\ME\Desktop\MixedInKeyTools\COMTest\build-QtCOMTest-Desktop_Qt_6_3_0_MinGW_64_bit-Debug\debug\QtCOMTest.exe crashed.

As you can see COMTestSubclass can be instanciated from C++, but not when returned from .net.
I tried using Interface, but same result.

What am i doing wrong ?

Thanks in advance for your help.

hskoglund

Hi, the cs file is most likely ok, but the dumpcpp utility that gives you the comtest.h file is a bit lacking when it comes to returning new class instances, so. the crash occurs because that class pointer is an orphan i.e. it tries to exist without belonging to a QAxObject.

One solution could be to create the COM object via a detour through a QAxObject and IDispatch, try this:

qInfo() << "SubClassPtr:";
auto subclassPtr = (COMTest::COMTestSubclass*) main->querySubObject("AGetSubclass");
qInfo() << subclassPtr->AGetString();
qInfo() << subclassPtr->ToString();

that should make sure that subclassPtr belongs to a QAxObhect and not crash :-)

Gundz

Hello,

That worked perfectly thank you !
I also saw and tested that i can give querySubObject parameters.

setProperty seems to be able to set a member variable (but it needs to be public in the cs).
But i don't see any GetProperty (which is not really a problem since i can make a getter).

But what is dynamicCall for ?

hskoglund

Hi, re. getProperty() you've spent too much time in Microsoft-land :-)
Qt uses the noun/setnoun standard, so for getting something:
subclassPtr->property("ToString");

Apart from property/setProperty there are two flavors of COM calls you can issue from your QAxObject classes:

dynamicCall() is like a superset of setProperty(), it's used when more than 1 argument is needed
querySubObject() is used mostly when navigating a hierarchy of COM objects, for example stepping dowm into an Excel spreadsheet.

(Also there's the propertyBag()/setPropertyBag() pair to get/set all the properties in one fell swoop, but I've never tried them)

Gundz

Actually i did not at all, it's the first time i'm doing C# (i need to open a SQL Ce Database to make a custom tool for a software and i don't want to learn the MS UI stuff ^^)

Thanks for the clarification !