Handle C# COM events in C++

Question

I have managed to create a C# COM object with events. Please find code below,

    [Guid("1212674-38748-45434")]
    public interface ICalculator
    {
        int Add(int Num1, int Num2);
    }

    [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
    [Guid("3453674234-84444-84784")]
    public interface ICalculatorEvents
    {
        [DispId(1)]
        void Completed(int Result);
    }

    [ClassInterface(ClassInterfaceType.None)]
    [ComSourceInterfaces(typeof(ICalculatorEvents))]
    [Guid("87457845-945u48-4954")]
    public class Calculator : ICalculator
    {
        public delegate void CompletedDelegate(int result);
        public event CompletedDelegate Completed;
        public Add(int Num1, int Num2)
        {
            int Result = Num1 + Num2;
            if(Completed != null)
                Completed(Result);
        }
    }

I have imported this COM object in a C++ console application and able to call the 'Add()' method. I am not sure how to handle 'Completed' event in my C++ application. Can you please advise on this? I am looking to display the result value in console whenever this event occurs.

Please find C++ application's code below. The Event 'Completed' never gets handled here. This goes into an infinite loop.

    #import "Calculator.tlb"
    using namespace Calculator;
    int Flag = 0;
    class HandleEvent : public ICalculatorEvent
    {
        public:
            HandleEvent(void);
            ~HandleEvent(void);
            HRESULT __stdcall QueryInterface(const IID &, void **);
            ULONG __stdcall AddRef(void) { return 1; }
            ULONG __stdcall Release(void) { return 1; }
            HRESULT __stdcall Completed(int Result);
    };

    HandleEvent::HandleEvent(void)
    {
    }

    HRESULT HandleEvent::Completed(int Result)
    {
        printf("Addition Completed, Result: %d", Result);
        Flag = 1;
    }

    HRESULT HandleEvent::QueryInterface(const IID & iid,void ** pp)
    {
        if (iid == __uuidof(ICalculatorEvent) || iid == __uuidof(IUnknown))
        {
            *pp = this;
            AddRef();
            return S_OK;
        }
        return E_NOINTERFACE;
    }

    int _tmain(int argc, _TCHAR* argv[])
    {
        CoInitialize(NULL);
        Flag = 0;
        ICalculatorPtr pCalc(__uuidof(Calculator));
        pCalc->Add(5, 6);

        do
        {
        }while(Flag == 0);

        CoUninitialize ();
        return 0;
    }

Thanks in advance.

I think you Completed event object won't get called because it is always null. Which class is implemented ICalculatorEvents interface ? — Kamran Amini
– Kamran Amini, Commented Jun 6, 2012 at 23:35
ICalculatorEvents is implemented in C++ application. Please find C++ code below, — GeekCandy
– GeekCandy, Commented Jun 6, 2012 at 23:40
sorry i couldnot add the code in the comment. so I have added this to my first post. Thanks — GeekCandy
– GeekCandy, Commented Jun 6, 2012 at 23:46
In _tmain() function, how pCalc->Add() should know which ICalculatorEvent implemented object should be called ? There is no EventHandler object here. If there was, in Add() method, Completed delegate doesn't point to any method. — Kamran Amini
– Kamran Amini, Commented Jun 6, 2012 at 23:52
I assumed that the method, HandleEvent::Completed(int Result) would be called when the event is invoked in Add method? I am completely new to COM and I think I am missing to understand something here. — GeekCandy
– GeekCandy, Commented Jun 7, 2012 at 0:03

Kamran Amini · Accepted Answer · 2012-06-07 00:13:02Z

0

If you want to use delegates you don't need to declare an interface. Change _tmain() function like this :

int _tmain(int argc, _TCHAR* argv[])
{
    CoInitialize(NULL);
    Flag = 0;

    EventHandler evh ;
    ICalculatorPtr pCalc(__uuidof(Calculator));
    pCalc->Completed = &evh.Completed() ;
    pCalc->Add(5, 6);

    do
    {
    }while(Flag == 0);

    CoUninitialize ();
    return 0;
}

If you want to use an interface try this.

 [ClassInterface(ClassInterfaceType.None)]
 [ComSourceInterfaces(typeof(ICalculatorEvents))]
 [Guid("87457845-945u48-4954")]
 public class Calculator : ICalculator
 {
     public ICalculatorEvents callbackObject ;

     public Add(int Num1, int Num2)
     {
         int Result = Num1 + Num2;
         if(callbackObject != null)
             callbackObject.Completed(Result);
     }
 }

and change _tmain() method to this.

int _tmain(int argc, _TCHAR* argv[])
{
    CoInitialize(NULL);
    Flag = 0;

    EventHandler evh ;
    ICalculatorPtr pCalc(__uuidof(Calculator));
    pCalc->callbackObject = &evh ;
    pCalc->Add(5, 6);

    do
    {
    }while(Flag == 0);

    CoUninitialize ();
    return 0;
}

answered Jun 7, 2012 at 0:13

Kamran Amini

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1 Comment

taiteilijaumbra Over a year ago

Should "EventHandler" be some global class or is it something you've defined yourself? VS2019 tells me "identifier is undefined". Do I need to include some header?

Aliaksei Plashchanski · Accepted Answer · 2013-04-19 08:18:57Z

I found that COM initialization in C++ client should be done using

    CoInitializeEx(NULL, COINIT_MULTITHREADED);

for asynchronous event processing from C# (.NET) COM server, otherwise C++ client receives events only after CoUninitialize() call.

Events handling class:

    class EventWrapper : public IDispEventSimpleImpl<1, EventWrapper, &DIID_RumCardCOMEvents >
    {
    public:
        // now you need to declare a sink map - a map of methods handling the events
        BEGIN_SINK_MAP(EventWrapper)
            SINK_ENTRY_INFO(1, DIID_RumCardCOMEvents, 0x1, isCardInserted, &cardInserted)
            SINK_ENTRY_INFO(1, DIID_RumCardCOMEvents, 0x2, isCardRemoved, &cardRemoved)
            // event interface id (can be more than 1)---+      |      |                   |
            // must match dispid of your event -----------------+      |                   |
            // method which handles the event  ------------------------+                   |
            // type information for event, see below --------------------------------------+
        END_SINK_MAP()

    // declare the type info object. You will need one for each method with different signature.
        // it will be defined in the .cpp file, as it is a static member
        static _ATL_FUNC_INFO cardInserted;  // 'placeholder' object to carry event information (see below)
        static _ATL_FUNC_INFO cardRemoved;  // 'placeholder' object to carry event information (see below)

        // method which handles the event
        STDMETHOD (isCardInserted)(unsigned char type)
        { 
            // usually it is defined it in the .cpp file
            cout << "isCardInserted: " << (int)type << endl;
            return 0;
        }

        STDMETHOD (isCardRemoved)()
        { 
            // usually it is defined it in the .cpp file
            cout << "isCardRemoved" << endl;
            return 0;
        }
    };

Main:

    int main()
    {
        CoInitializeEx(NULL, COINIT_MULTITHREADED);
        try
        {
            EventWrapper ev;
            ev.DispEventAdvise(/*COM interface*/);
            // receiving events
            ev.DispEventUnadvise(/*COM interface*/);
        }
        catch (_com_error& e)
        {
            cout << "Exception: " << e.ErrorMessage() << endl;
        }

        CoUninitialize();
        return 0;
    }

What is "&DIID_RumCardCOMEvents"? Is RumCardCOMEvents your COM event interface? Where does the "DIID" part come from?
Yes it is COM event interface, sorry don't remember details, it was too long ago.

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