Issue with QueryInterface

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Hi All,
I have tried to use QueryInterface by using the following code segment. the com component had been created by me and which has two interfaces. But the "queryinterface" returns E_NOINTERFACE.
Kindlly help me, to come out of this issue.
Thanks,
Ganesamoorthy

C++

 HRESULT hr;
IGreet *greetObject;
IGreet2 *greetObject2;
CString strTemp;
hr = CoInitialize(NULL);
if (FAILED(hr) )
    return ;
hr = CoCreateInstance(CLSID_Greet2,NULL,CLSCTX_INPROC_SERVER, IID_IGreet2,(void **) & greetObject2);
if (FAILED(hr) )
{
    CoUninitialize();
    return ;
}
greetObject2->AddRef();
cout << greetObject->Sayhello("Ganesh") << endl;
CComBSTR bstrVal, bstrVal1;

bstrVal1.Append ("Ganesh");
greetObject2->SayHello2(bstrVal1, & bstrVal);
strTemp.Format (_T("%s"), bstrVal);
MessageBox (strTemp);
hr = greetObject2->QueryInterface(IID_IGreet,(void **)& greetObject);
DWORD dw = GetLastError();
if (FAILED(hr) )
{
    CoUninitialize();
    return ;
}
CoUninitialize();

Posted 26-Jan-11 22:47pm

Ganesamoorthy.S

Updated 27-Jan-11 0:12am

CPallini

v2

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Comments

Ryan Zahra 27-Jan-11 5:04am

Is IGreet registered correctly? How are you registering it?
Is IGreet2 inheriting from IGreet?

Ganesamoorthy.S 27-Jan-11 5:55am

During build of the project the component is getting registered properly.

And i am able to get any one of the objects using createinstance. But i am unable to use Queryinterface.

IGreet2 is not inherited from IGreet.

Ganesamoorthy.S 27-Jan-11 5:56am

During build of the project the component is getting registered properly.

And i am able to get any one of the objects using createinstance. But i am unable to use Queryinterface.

IGreet2 is not inherited from IGreet.

Ganesamoorthy.S 27-Jan-11 6:31am

I am a beginner in COM, can you please clarify me?

I am using VS2005. How can i inherit the interface? Is there any wizard to do this?

Ryan Zahra 27-Jan-11 6:37am

It's difficult to explain COM in a few lines. Here's a good tutorial to get you started:
http://www.codeproject.com/kb/com/CCOMThread.aspx#COM_APARTMENTS
http://www.codeproject.com/KB/COM/CCOMThread2.aspx

Also I recommend reading the book Inside COM by Dale Rogerson which is very simple to read and understand. I've learned COM from that book.

Ganesamoorthy.S 27-Jan-11 6:40am

Thanks a lot

Ganesamoorthy.S 28-Jan-11 7:01am

AcceptÂ Answer RejectÂ Answer
.I have completed this on my own.

Adding interface to a COM, is as usual but if we suppose to use QUERYINTERFACE then it has to be implemented(implement interface option must be chosen). As you have instructed me in the threads it has to be inherited.

I did it and got the expected result.

Ryan Zahra 28-Jan-11 9:00am

Good to hear :)

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Ryan Zahra · Accepted Answer · 2011-01-27T00:05:00

Your problem is that IGreet2 is not inherited from IGreet. You must inherit IGreet in IGreet2. In the QueryInterface of IGreet2, you must create an instance of IGreet.

C

HRESULT __stdcall Greet2::QueryInterface(REFIID riid, void **ppObj)
{
    if(riid == IID_IUnknown || riid == IID_IGreet2)
    {
        *ppObj = static_cast<IGreet2*>(this) ;
        reinterpret_cast<IUnknown*> (*ppObj)->AddRef();
        return S_OK;
    }
    else if(riid == IID_Greet)
    {
        *ppObj = static_cast<IGreet*>(this) ;
        reinterpret_cast<IUnknown*> (*ppObj)->AddRef();
        return S_OK;
    }
    else
    {
        *ppObj = NULL;
        return E_NOINTERFACE;
    }
}