Question

I’ve created a COM library defined in an IDL, it has an “entrypoint” called IFoo:

[
    object,
    uuid(789b4d46-4028-4196-8412-4c5c8ef86caa),
    nonextensible,
    pointer_default(unique)
]
interface IFoo: IUnknown
{
    HRESULT HelloWorld();
};

I’ve implemented this in my C# library like so:

[ComVisible(true)]
[Guid("45b50f1e-d551-4be0-b52a-7ec075840114")]
[ClassInterface(ClassInterfaceType.None)]
[ComDefaultInterface(typeof(IFoo))]
class Foo: IFoo
{
    public void HelloWorld()
    {
        Console.WriteLine("Hello world");
    }
}

I compiled this, and registered it with:

regasm.exe foo.dll

Everything looks good thus far.

Now, how do I create an instance of this in my C++ program? I think the right function is ::CoCreateInstance:

::CoInitializeEx(NULL, COINIT_MULTITHREADED); 
IUnknown *pUnk = 0;
HRESULT hr = ::CoCreateInstance(???, 
                                NULL, 
                                CLSCTX_INPROC_SERVER, 
                                __uuidof(IFoo), 
                               (void**)&pUnk);

Is this the right way to create an instance of this class? If so, what goes in “???” above?

I think the class’s UUID (“45b50f1e-d551-4be0-b52a-7ec075840114” above) should go there, but I can’t figure out how to manually create an IID from this GUID string.

Answer 1

It turns out, I was right, the class’s UUID goes there, but getting it was something I didn’t expect:

struct __declspec(uuid("45b50f1e-d551-4be0-b52a-7ec075840114")) Cls;

Then:

HRESULT hr = ::CoCreateInstance(__uuidof(Cls), 
                                NULL, 
                                CLSCTX_INPROC_SERVER, 
                                __uuidof(IFoo), 
                               (void**)&pUnk);

This fixed the problem and worked correctly.