Q272676: BUG: CComContainedObject::QueryInterface() Implementation Is Inc

Article: Q272676
Product(s): Microsoft C Compiler
Version(s): 3.0
Operating System(s): 
Keyword(s): kbATL kbDSupport kbGrpDSMFCATL
Last Modified: 08-MAY-2002

-------------------------------------------------------------------------------
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 <AtlCom.h>
      #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. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.