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Q185674: PRB: Default Property Causes DISP_XXXX_ID Methods/Props to Fail

Article: Q185674
Product(s): Microsoft C Compiler
Version(s): winnt:5.0
Operating System(s): 
Keyword(s): kbole kbActiveX kbAutomation kbCtrl kbMFC kbVC500 kbGrpDSMFCATL kbNoUpdate
Last Modified: 17-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 Enterprise Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

When using DISP_DEFVALUE to specify a default property in an MFC IDispatch
interface, properties and methods declared with one of the DISP_XXXX_ID macros
fails if the DISPID specified is one greater than the last ID generated by MFC.

CAUSE
=====

MFC maintains a table of dispatch entries, which it searches when methods and
properties are accessed through IDispatch::Invoke. Because items placed in the
dispatch map without using DISP_XXXX_ID macros do not have DISPIDs associated
with them internally, MFC relies on the order in which they are placed in the
table.

DISP_DEFVALUE causes an extra entry to be placed in the table before any of the
entries specified with the DISP_XXXX_ID macro. If the DISP_XXXX_ID macro is
using a DISPID exactly 1 greater than the last entry, the default property is
invoked instead of the one specified with the DISP_XXXX_ID macro.

RESOLUTION
==========

Remove the default property, or specify a DISPID in the DISP_XXXX_ID macro that
is at least 2 greater than the number of properties and methods in the dispatch
map not specified using the DISP_XXXX_ID macros.

For example, when using a default property, any DISPIDs used in DISP_XXXX_ID
macros should be 2 greater than the total number of entries in the dispatch map
minus the number specified with DISP_XXXX_ID:

  2 + (# of Dispatch Map entries) - (# of DISP_XXXX_ID entries) = DISPID of
  first DISP_XXXX_ID macro

If you need to specify a default property and assign to a property or method a
low value DISPID, you may need to use DISP_XXXX_ID macros for all of the methods
and properties in the dispatch map.

STATUS
======

This feature is by design.

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

Although ClassWizard maintains dispatch IDs for properties and methods in the
.odl file, DISPIDs are not specified for entries in the dispatch map unless they
are specified manually using one of the DISP_XXXX_ID macros:

  DISP_FUNCTION_ID
  DISP_PROPERTY_ID
  DISP_PROPERTY_NOTIFY_ID
  DISP_PROPERTY_EX_ID
  DISP_PROPERTY_PARAM_ID

REFERENCES
==========

For more information on MFC's dispatch maps, DISP macros, and default DISPID
numbering, see MFC Technote #39 in the Visual C++ documentation.
(c) Microsoft Corporation 1998, All Rights Reserved.
Contributions by Kelly Marie Ward, Microsoft Corporation


Additional query words: automation

======================================================================
Keywords          : kbole kbActiveX kbAutomation kbCtrl kbMFC kbVC500 kbGrpDSMFCATL kbNoUpdate 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:5.0
Issue type        : kbprb
Solution Type     : kbnofix

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