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

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.

LCC · Accepted Answer · 2012-05-02 02:13:34Z

2

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.

answered May 2, 2012 at 2:13

LCC

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Comments

Pavel Zhuravlev · Accepted Answer · 2012-05-02 02:12:46Z

1

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.

answered May 2, 2012 at 2:12

Pavel Zhuravlev

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4 Comments

LCC Over a year ago

I did use MIDL to compile down to a set of C++ files. I used this to produce a .lib I link against and a .h I include. I think you missed the point of the question, how to activate and create an instance of this class I made in .Net. Unless, of course, I'm misunderstanding your reply. =)

Pavel Zhuravlev Over a year ago

@LCC One of the files created by MIDL (namely your_lib_name_i.c) contains the definitions of all GUIDS in project and your_lib_name.h has the declarations of these GUIDS. By including these files into your C++ you will be able to use constants like CLSID_Foo in your COM functions calls.

Pavel Zhuravlev Over a year ago

You may have missed the CLSID of your class in your MIDL-generated files because your IDL is missing the class declaration

LCC Over a year ago

Ah, that makes sense. I didn't define the class there, so that's part of the problem. Thanks!

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