How to get interface to C# object wrapped in COM, from MFC DLL

Question

I have C# object in a DLL. I call a method of this object via COM from MFC DLL. like this:

BOOL CForwarder::InitMyManagedFlex()
{
    // Initialize COM.
    HRESULT hr = CoInitialize(NULL);

    // Create the interface pointer.
    IForwarderPtr pIFwd(__uuidof(MyForwarder));

    long lResult = 0;
    VARIANT_BOOL ret = FALSE;
    BSTR bstr = AsciiToBSTR("AAA");

    // Call the Add method.
    pIFwd->Start(bstr, &ret);

    SysFreeString(bstr);

    wprintf(L"The result is %d\n", ret);

    // Uninitialize COM.
    CoUninitialize();

    return (ret == VARIANT_TRUE) ? TRUE : FALSE;
}

Then after a while I call another method SetTimeFormat in exactly the same manner. The problem is that every time I call in this manner I instantiate a new C# object, but in fact I am trying to reach the same object that was created the first time and simply set one of its properties to a different value.

The problem seems to be in this line:

IForwarderPtr pIFwd(__uuidof(MyForwarder));

So how do I get the interface to the same C# object instead of creating a new one?

Answer 1

// Create the interface pointer.
IForwarderPtr pIFwd(__uuidof(MyForwarder));

Actually that line does two things. It creates the COM object whose UUID/ClassID is MyForwarder and stores the result in pIFwd .

If you don't want to create it each time try this:

IForwarderPtr pIFwd(__uuidof(MyForwarder));
pIFwd->Start(bstr, &ret);
.
.
.
.
pIFwd->SetTimeFormat (....)

.
.
.
SysFreeString(bstr);

wprintf(L"The result is %d\n", ret);

// Uninitialize COM.
CoUninitialize();

Answer 2

You can move the following statements to your constructor

HRESULT hr = CoInitialize(NULL);
_pIFwd(__uuidof(MyForwarder)); //where _pIFwd is a member variable of your class

& the statement CoUninitialize() to your destructor. Add IForwardPtr as a member variable of your class.

This way you will be able to re-use the same instance.