How to create a HelloWorld COM Interop in Visual Studio 2012

Question

First off, I am new to COM, and currently quite confused. I\'ve read a lot of documentation on COM on MSDN and the general web, but a lot of it seems outdated and overly com

Answer 1

Try following these steps:

1. Make sure both projects, unmanaged C++ and managed C# have the same bitness, either x86 or x64. Let's say it's x86, for clarity.
2. Open Admin Command Prompt and register your COM DLL: C:\Windows\SysWOW64\regsvr32.exe c:\full-path\ComLib.Interop.dll
3. Run Visual Studio as Admin. Do steps 1,2,4,5,6. Don't do 3.

See if you get to 7. I think that should work.

Note you only need the registration on the Development machine. Isolated COM should work everywhere else.

Answer 2

You probably went wrong at step #2, given that you didn't get a build error. The wizard gives you more than one choice for the kind of class you add. The default choice is "C++ class", you need to pick ATL + "ATL Simple Object" instead.

The Class View window now shows two types getting added, the IInteropDemo interface and the CInteropDemo class that implements the interface. You next right-click the interface type (not the class) and use "Add Method". You can now also take a look at the IDL file in the project, it should resemble this:

import "oaidl.idl";
import "ocidl.idl";

[
    object,
    uuid(CBA0D899-2F4C-4F1D-A935-C80CB981C153),
    dual,
    nonextensible,
    pointer_default(unique)
]
interface IInteropDemo : IDispatch{
    [id(1)] HRESULT Method();
};
[
    uuid(ED14ACED-4FF9-4144-B302-CC48C481F28B),
    version(1.0),
]
library ATLProject4Lib
{
    importlib("stdole2.tlb");
    [
        uuid(8543642F-9927-451C-9784-6A7774418299)      
    ]
    coclass InteropDemo
    {
        [default] interface IInteropDemo;
    };
};

That's enough to get it built. Which ought to fail on any modern Windows version, UAC prevents the COM server getting registered. Which requires step #0: Start Visual Studio by right-clicking the shortcut and selecting "Run as Administrator".