Using COM Interop Objects from C++

Question

I have a .NET Library registered for COM Interop which I successfully use from VB6 code (so, it's propperly registered and this is not the issue).

What happens is that I'm trying to use this same library via C++ code (an MFC ActiveX Control) and I'm not even being able to instantiate the object (yes, it has a parameterless constructor).

Imagine that the namespace is Foo and the name of the class is Bar. How would I instantiate such object? I'm trying this:

On the .h file:

Foo::Bar *obj;

On the .cpp file, on the constructor of the ActiveX control:

obj = new Foo::Bar();

And I'm getting the following errors:

«incomplete type is not allowed"»;
«error C2027: use of undefined type 'Foo::Bar'».

So basically my question is: How does one use .NET COM Interop objects from C++ code?

You are not close to writing correct C++ code. Start with the #import directive to get closer, a book will certainly not hurt either. — Hans Passant
– Hans Passant, Commented Jul 30, 2013 at 18:59
I'm using the #import directive, obviously. I've omitted it because I thought it would be obvious for everyone. I'm familiar with C++, not with Windows or COM Interop. — amgcgoncalves
– amgcgoncalves, Commented Jul 31, 2013 at 10:12

Eric Finn · Accepted Answer · 2013-07-31 13:01:53Z

2

CComObject<Foo::Bar>* obj;
HRESULT hr = CComObject<Foo::Bar>::CreateInstance(&obj);
//check hr

Or perhaps if you want to use an interface called IBar that Bar implements:

CComPtr<Foo::IBar> pIObj;
HRESULT hr = pIObj.CreateInstance(__uuidof(Foo::Bar));
//check hr

And as Hans suggested in his comment, you're going to want to use the #import directive to import the type library.

edited Jul 31, 2013 at 13:01

answered Jul 30, 2013 at 19:26

Eric Finn

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2 Comments

amgcgoncalves Over a year ago

Thanks a lot, Eric! I've been successful with your second suggestion (using an interface and a type implementing it).

Eric Finn Over a year ago

@amgcgoncalves Good to hear! And good luck with working with COM. It can be a bit tough to deal with if you're not used to it.

Matt · Accepted Answer · 2013-07-31 14:44:20Z

1

in C++, first you should initialize the COM, then get the COM object through UUID, the you can use the function of this COM object, after it is done, uninitialize the COM. So the the code will be like this:

CoInitialize(NULL);

IMyClassPtr myClass = IMyClassPtr(_uuid(myClass));
if (myClass != NULL)
   {
     myClass->DoSomething();
 }

 CoUnitialize();

answered Jul 31, 2013 at 14:44

Matt

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