CoCreateInstance: Create a COM instance defined in a .NET library? - c++

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.

Answer 2

Just use MIDL to compile your IDL file into set of C++ files and include them into your C++ program. Have in mind that interface is not the "entry point" and you are going to need the class object in your IDL too.
Another way to create a COM client in C++ is Microsoft specific #import directive, but I am not sure if it is compatible with .NET-based components. If you can create a .tlb file for your .NET component, you can just add the needed stuff like this:
#import "MyLibrary.tlb"
This directive has lots of options. Check this MSDN article for details.