Q133042: HOWTO: Pass IDispatch Pointer and Avoid an Application Error

Article: Q133042
Product(s): Microsoft C Compiler
Version(s): winnt:2.0,2.1,2.2,4.0,4.1,5.0,6.0
Operating System(s): 
Keyword(s): kbActiveX kbAutomation kbCOMt kbMFC kbServer kbVC200 kbVC400 kbVC500 kbVC600 kbGrpDSMFC
Last Modified: 30-JUL-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0, 4.1 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SUMMARY
=======

In some cases, you may want an OLE Automation Local-Server to pass its IDispatch
pointer to another Local-Server through an automation method. However, the
pointer will become invalid after the method in the second server returns, so an
Access Violation will occur if the second server tries to use the pointer
later.

To maintain the integrity of the pointer, you need to call AddRef on the
IDispatch pointer within the second server, as described in the following rule
from the OLE SDK documentation:

  If a local copy of an interface pointer is made from an existing global
  interface pointer, the local copy must be independently reference-
  counted. This separate reference count is necessary because the global
  copy can be destroyed while the local copy is in use.

MORE INFORMATION
================

Here is an example demonstrating the reasoning behind the general rule.

When the IDispatch pointer from the first Local-Server (server1) is passed to the
second Local-Server (server2), the RPC manager creates a proxy in the address
space of server2. The pointer that server2 actually receives points to the
proxy. This should make sense because the original object resides in the address
space of server1, so server2 cannot actually have a pointer to it.

The default life-time of the proxy is the duration of the function or automation
method. In many cases, however, it would be convenient to use the pointer later.
In MFC, you can do this by creating an instance of COleDispatchDriver (or an
instance of an object derived from COleDispatchDriver) and calling
AttachDispatch().

To affect the life-time of the proxy, server2 must call AddRef for the automation
object it receives. The RPC Manager will intercept this call and increment the
reference count on both the proxy and the object. It is important to note that
AddRef must be called from server2 because the proxy lives in server2's address
space.

The following sample code demonstrates how to do it. It is assumed that
CServer2Doc is an automation object. The CServerDoc2::SetReturnDispatch method
receives a LPDISPATCH object from server1 and will attach it to
CServer2Doc::m_DDServer1, which is a instance of an object derived from
COleDispatchDriver. Note that m_DDServer1 will be deleted when CServer2Doc is
deleted; Because AttachDispatch is called with the default second parameter,
this will cause the LPDISPATCH object Release to be called.

Refer to COleDispatchDriver::~COleDispatchDriver and
COleDispatchDriver::AttachDispatch for more information.

Sample Code
-----------

  void CServer2Doc::SetReturnDispatch(LPDISPATCH lpDispServer1)
  {
    lpDispServer1->AddRef();         // AddRef so it can be used later
    m_DDServer1.AttachDispatch(lpDispServer1);
  }

REFERENCES
==========

- Visual C++ Books Online Contents\OLE 2.0 SDK\Chapter 6 Component Object
  Interfaces and Functions \Iunknown Interface\Reference Counting Rules.

- OLE 2 Programmers Reference, Volume 1, pages 191-195.

- Inside OLE 2, pages 83-90.

Additional query words:

======================================================================
Keywords          : kbActiveX kbAutomation kbCOMt kbMFC kbServer kbVC200 kbVC400 kbVC500 kbVC600 kbGrpDSMFCATL 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:2.0,2.1,2.2,4.0,4.1,5.0,6.0
Issue type        : kbhowto

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