QueryInterface -- How come this code doesn't work?

[Popeye Doyle Murray]

Here is the code:

// this line gets a valid ADO database connection object
_ConnectionPtr pConnection = m_AdoDatabase.GetActiveConnection();

// Now I will get a pUknown pointing to the database connection
IUnknown* pUnk;

// this is successful
HRESULT hr = pConnection->QueryInterface(IID_IDispatch, (void**) &pUnk);

// Now I will attempt to get an ADO connection object from pUnknown
_ConnectionPtr pConnection2;

// this line fails
hr = pConnection->QueryInterface(__uuidof(Connection), (void**) &pConnection2);

if (FAILED(hr))
{
// this is where the failure is!
// hr is returned with a value E_NOINTERFACE
TRACE("Failed to get pointer the second time.\n");
TRACE("Error: %s\n\n", GetComError(hr));
}

Shouldn't I be able to go from an ADO Connection object, to IUnknown, and then back again? How do I get back my ADO Connection from IUnkown?

[Stephane Rodriguez]

Haven't checked, but may be __uuidof(Connection) returns the CLSID of the Connection object, while what is expected in queryinterface() is the IID of the interface.

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[geo_m]

right!

[Steve S]

And as an aside, you're not asking for IUnknown in the first QueryInterface, but IDispatch. In the case you have, both should work, of course... Steve S [This signature space available for rent]

[Popeye Doyle Murray]

So how do I get the IID?

[Stephane Rodriguez]

I believe either __uuidof(_Connection) or __uuidof(_ConnectionPtr) is better. Make sure to check that the return IID is non-NULL by the way.

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