Events in .NET are seen as Connection Point on native side. You can use them with ATL as described here ATL Connection Points and Event Handling Principles

So here's a small recap.

Here is your C# class

namespace Eggplant
{
    [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
    [Guid("C61C7C47-BB98-4DF3-BC61-7CA9430EDE7A")]
    [ComVisible(true)]
    public interface IEggplantClientEvents
    {
        [DispId(1)]
        void Completed(string text);
    }

    [Guid("0a805b99-756a-493c-96b7-063400f171ed")]
    [ComVisible(true)]
    [ClassInterface(ClassInterfaceType.None)]
    [ComSourceInterfaces(typeof(IEggplantClientEvents))]
    [ProgId("EggplantClient.CEggplantClient")]
    public class CEggplantClient
    {
        [ComVisible(false)] public delegate void CompletedDelegate(string text);
        public event CompletedDelegate Completed;

        public CEggplantClient()
        {
            // wait 2 seconds and then call every second
            Task.Delay(2000).ContinueWith(async t =>
            {
                do
                {
                    Completed?.Invoke("Time is " + DateTime.Now);
                    await Task.Delay(1000);
                }
                while (true);
            });
        }
    }
}

You can register your C# class like this (will create an Eggplant.tlb file) with .NET Framework:

%windir%\Microsoft.NET\Framework64\v4.0.30319\RegAsm.exe Eggplant.dll /codebase /tlb

note: with .NET core and .NET 5/6/7+ you'll have to build your own .TLB, or copy your C# .NET Core code into a .NET Framework .dll and use this ...

Here is your C/C++ code (forward refs omitted):

#include <windows.h>
#include <stdio.h>
#include <atlbase.h>
#include <atlcom.h>

#import "D:\kilroy\was\here\Eggplant\bin\Debug\Eggplant.tlb" // import the tlb

using namespace Eggplant; // #import by default puts generated code in a specific namespace

int main()
{
    CoInitialize(nullptr);
    {
        CComPtr<IUnknown> app;
        if (SUCCEEDED(app.CoCreateInstance(__uuidof(CEggplantClient))))
        {
            // sink events
            auto sink = new CEggplantClientEventsSink();
            if (SUCCEEDED(sink->Connect(app)))
            {
                // this message box allows us to wait while events arrive
                MessageBox(nullptr, L"Click to stop listening", L"Events", MB_OK);
            }
        }
    }
    CoUninitialize();
    return 0;
}

// this is the event sink
class CEggplantClientEventsSink : public CDispInterfaceBase<IEggplantClientEvents>
{
public:
    CEggplantClientEventsSink() { }

    HRESULT Invoke(DISPID dispid, DISPPARAMS* pdispparams, VARIANT* pvarResult)
    {
        switch (dispid)
        {
        case 1: // the Completed DISPID value
            wprintf(L"Completed called text:%s\n", pdispparams->rgvarg[0].bstrVal);
            break;
        }
        return S_OK;
    }
};

// this is a generic support class to hook IDispatch events
// adapted from here: https://devblogs.microsoft.com/oldnewthing/20130612-00/?p=4103
template<typename DispInterface>
class CDispInterfaceBase : public DispInterface
{
    LONG m_cRef;
    CComPtr<IConnectionPoint> m_spcp;
    DWORD m_dwCookie;

public:
    CDispInterfaceBase() : m_cRef(1), m_dwCookie(0) { }

    // IUnknown
    IFACEMETHODIMP QueryInterface(REFIID riid, void** ppv)
    {
        *ppv = nullptr;
        HRESULT hr = E_NOINTERFACE;
        if (riid == IID_IUnknown || riid == IID_IDispatch || riid == __uuidof(DispInterface))
        {
            *ppv = static_cast<DispInterface*>(static_cast<IDispatch*>(this));
            AddRef();
            hr = S_OK;
        }
        return hr;
    }

    IFACEMETHODIMP_(ULONG) AddRef() { return InterlockedIncrement(&m_cRef); }
    IFACEMETHODIMP_(ULONG) Release() { LONG cRef = InterlockedDecrement(&m_cRef); if (!cRef) delete this; return cRef; }

    // IDispatch
    IFACEMETHODIMP GetTypeInfoCount(UINT* pctinfo) { *pctinfo = 0; return E_NOTIMPL; }
    IFACEMETHODIMP GetTypeInfo(UINT iTInfo, LCID lcid, ITypeInfo** ppTInfo) { *ppTInfo = nullptr; return E_NOTIMPL; }
    IFACEMETHODIMP GetIDsOfNames(REFIID, LPOLESTR* rgszNames, UINT cNames, LCID lcid, DISPID* rgDispId) { return E_NOTIMPL; }
    IFACEMETHODIMP Invoke(DISPID dispid, REFIID riid, LCID lcid, WORD wFlags, DISPPARAMS* pdispparams, VARIANT* pvarResult, EXCEPINFO* pexcepinfo, UINT* puArgErr)
    {
        if (pvarResult) VariantInit(pvarResult);
        return Invoke(dispid, pdispparams, pvarResult);
    }

    virtual HRESULT Invoke(DISPID dispid, DISPPARAMS* pdispparams, VARIANT* pvarResult) = 0;

public:
    HRESULT Connect(IUnknown* punk)
    {
        CComPtr<IConnectionPointContainer> spcpc;
        HRESULT  hr = punk->QueryInterface(IID_PPV_ARGS(&spcpc));
        if (SUCCEEDED(hr)) hr = spcpc->FindConnectionPoint(__uuidof(DispInterface), &m_spcp);
        if (SUCCEEDED(hr)) hr = m_spcp->Advise(this, &m_dwCookie);
        return hr;
    }

    void Disconnect()
    {
        if (m_dwCookie)
        {
            m_spcp->Unadvise(m_dwCookie);
            m_spcp.Release();
            m_dwCookie = 0;
        }
    }
};

And this is the result: