DOCUMENT:Q272676 08-MAY-2002 [visualc] TITLE :BUG: CComContainedObject::QueryInterface() Implementation Is Inc PRODUCT :Microsoft C Compiler PROD/VER::3.0 OPER/SYS: KEYWORDS:kbATL kbDSupport kbGrpDSMFCATL ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - The Microsoft Active Template Library (ATL) 3.0 ------------------------------------------------------------------------------- SYMPTOMS ======== When you are writing an aggregatable COM object in Active Template Library (ATL), unintended inner object interfaces could be exposed because of a bug in the implementation of the CComContainedObject::QueryInterface function. CAUSE ===== The CComContainedObject::QueryInterface function calls the following lines in AtlCom.h: STDMETHOD(QueryInterface)(REFIID iid, void ** ppvObject) { HRESULT hr = OuterQueryInterface(iid, ppvObject); if (FAILED(hr) && _GetRawUnknown() != m_pOuterUnknown) hr = _InternalQueryInterface(iid, ppvObject); return hr; } If the OuterQueryInterface call fails, a "blind" _InternalQueryInterface is performed in the case of an aggregated object. This can cause a problem when, for instance, the outer object does not want to expose an interface of the inner object (and returns, E_FAIL, for example). The "blind" _InternalQueryInterface in the code returns that interface anyway. RESOLUTION ========== To work around the problem and enable the aggregation to work correctly, comment out the highlighted lines in the AtlCom.h file: STDMETHOD(QueryInterface)(REFIID iid, void ** ppvObject) { HRESULT hr = OuterQueryInterface(iid, ppvObject); // Comment out the two lines below: // if (FAILED(hr) && _GetRawUnknown() != m_pOuterUnknown) // hr = _InternalQueryInterface(iid, ppvObject); return hr; } Make these modifications to a copy of the AtlCom.h file (for instance, FixAtlCom.h). Then, in the Stdafx.h file, comment out AtlCom.h and use FixAtlCom.h instead: // #include #include "FixAtlCom.h" STATUS ====== Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. MORE INFORMATION ================ ATL uses CComContainedObject in the CComAggObject, CComPolyObject, and CComCachedTearOffObject classes. CComContainedObject implements IUnknown by delegating to the owner object's IUnknown. (The owner is either the outer object of an aggregation or the object for which a tear-off interface is being created.) To prevent unintended side effects and to allow the owner object to have full control, CComContainedObject must not independently return its own interfaces in response to a QueryInterface request, if the owner object's QueryInterface (called through OuterQueryInterface) does not work correctly. For example, assume that an inner object has an interface IID_Moo, which an outer object does not want to expose. The outer object could use a COM_INTERFACE_ENTRY_NOINTERFACE(IID_Moo) macro in its ATL COM map, right before a COM_INTERFACE_ENTRY_AGGREGATE_BLIND macro. The current ATL code for CComContainedObject::QueryInterface will allow a QueryInterface for IID_Moo to succeed, which is not expected behavior. The changes given here will allow for the IID_Moo QueryInterface call to fail, as expected. REFERENCES ========== For additional information, click the article number below to view the article in the Microsoft Knowledge Base: Q181265 HOWTO: Create ATL COM Objects View the following topics in the Microsoft Developer Network (MSDN) library: CComContainedObject http://msdn.microsoft.com/library/devprods/vs6/visualc/vcmfc/_atl_ccomcontainedobject.htm CComAggObject http://msdn.microsoft.com/library/devprods/vs6/visualc/vcmfc/_atl_ccomaggobject.htm CComPolyObject http://msdn.microsoft.com/library/devprods/vs6/visualc/vcmfc/_atl_ccompolyobject.htm ATL Macros and Global Functions http://msdn.microsoft.com/library/devprods/vs6/visualc/vcmfc/_atl_atl_macros_and_global_functions.htm Additional query words: ====================================================================== Keywords : kbATL kbDSupport kbGrpDSMFCATL Technology : kbVCsearch kbAudDeveloper kbATLsearch kbATL300 Version : :3.0 Issue type : kbbug ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. 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