How to wrap a thirdparty library COM class for notifications in a C++ project

Question

A thirdparty vendor has provided a COM interface as an external API to their product, and for sending instructions to it, it's fine with the mechanism of generating class wrappers with visual studio by importing from the dll, then creating instances of the created classes, calling their CreateDispatch(), using the access functions and releasing when done.

For return notifications, I'd normally create an EventsListener class derived from IDispatch using the Invoke function to handle events returning from the interface.

This vendor has created an Events class which I have to wrap and expose, then explicitly tell the installation where to look. all the example are given is C# where it's really easy, but I'm struggling on how to do it in C++

in the C# example provided with the API, the interop dll provided is simply added as a reference and derived into a class like so:

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;
using System.Text;

using <THIER INTEROP LIB>

namespace some.example.namespace
{
    [ComVisible(true)]
    public class EventViewer : IEvents //where IEvents is their events class
    {
        public void OnEvent(EventID EventID, object pData) //overridden function
        {
            //event handled here
        }
    }
}

In broad terms I assume that I must create a COM interface, since they require a ProgID from me to instantiate, but how do I derive that's been wrapped by the import and then expose the created class to COM

I'm just not sure where to even start, as all the tutorials I've seen so far talk in terms of creating brand new classes not wrapping a third party one

Answer 1

So they have provided you with an interface to implement and they get hold of your implementation by CoCreating the progid that you give to them? That's one way to do it, I guess...

How do you communicate your progid to their code? What expectations do they have of the COM class that you need to create?

You could either create an ATL COM object (dll) and simply implement the interface they give you (I assume they have either given you an idl file or a tlb?). However that will allow them to create your dll inside their process and it may not be ideal for you (how does that com dll then communicate with your process?, etc.) Alternatively you might look at creating an exe server rather than a dll. This would mean that you would run your process, register its objects, control the third-party app via its interface, pass it your prog id and then it could call back into your process... Again, ATL COM tutorials should help here.