The dispatch identifiers change when you add properties to existing Automation objects by using ClassWizard

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This article was previously published under Q179691

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SYMPTOMS

When adding properties by using ClassWizard to existing Automation objects, the dispatch identifiers (DISPIDs) of any methods will change.

Although changing DISPIDs does not usually cause a problem when originally building an Automation server, it can cause incompatibility problems when updating existing Automation objects. Automation clients that were built using early binding will no longer work with the updated server.

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CAUSE

ClassWizard adds properties to MFC's "dispatch map" before methods. Because, by default, DISPIDs are assigned to interfaces based on the order in which they are added to the map, adding a property changes the IDs of all the existing methods.

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RESOLUTION

Add properties and methods to the dispatch map and the .odl file without using ClassWizard.

You should add these additional properties outside the ClassWizard comments at the end of the dispatch map.

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STATUS

Microsoft is researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

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MORE INFORMATION

There are several MFC dispatch DISP_XXXX_ID macros that allow you to specify a specific DISPID:

   DISP_FUNCTION_ID()
   DISP_PROPERTY_ID()
   DISP_PROPERTY_NOTIFY_ID()
   DISP_PROPERTY_EX_ID()
   DISP_PROPERTY_PARAM_ID()

These macros can be useful in maintaining existing DISPID numbering.

Here is an sample dispatch map from a sample control:

//////////////////////////////////////////////////////////////////////// 
   // Dispatch map:
   BEGIN_DISPATCH_MAP(CTestCtrl, COleControl)
      //{{AFX_DISPATCH_MAP(CTestCtrl)
      DISP_PROPERTY_NOTIFY(CTestCtrl, "Property1", m_property1,
                           OnProperty1Changed, VT_I2)
      DISP_PROPERTY_EX(CTestCtrl, "Property2", GetProperty2,
                       SetProperty2, VT_I2)
      DISP_FUNCTION(CTestCtrl, "Method1", Method1, VT_I2, VTS_I2)
      DISP_FUNCTION(CTestCtrl, "Method2", Method2, VT_I4, VTS_NONE)
      //}}AFX_DISPATCH_MAP
      DISP_FUNCTION_ID(CTestCtrl, "AboutBox", DISPID_ABOUTBOX,
                      AboutBox, VT_EMPTY, VTS_NONE)
   END_DISPATCH_MAP()

Here is the same dispatch map using the DISP_XXX_ID macros to specify specific DISPIDs:

//////////////////////////////////////////////////////////////////////// 
   // Dispatch map:
   BEGIN_DISPATCH_MAP(CTestCtrl, COleControl)
      //{{AFX_DISPATCH_MAP(CTestCtrl)
      //}}AFX_DISPATCH_MAP
      DISP_PROPERTY_NOTIFY_ID(CTestCtrl, "Property1", 1,
                              m_property1, OnProperty1Changed, VT_I2)
      DISP_PROPERTY_EX_ID(CTestCtrl, "Property2", 2, GetProperty2,
                          SetProperty2, VT_I2)
      DISP_FUNCTION_ID(CTestCtrl, "Method1", 3, Method1, VT_I2, VTS_I2)
      DISP_FUNCTION_ID(CTestCtrl, "Method2", 4, Method2, VT_I4, VTS_NONE)
      DISP_FUNCTION_ID(CTestCtrl, "AboutBox", DISPID_ABOUTBOX, AboutBox,
                       VT_EMPTY, VTS_NONE)
   END_DISPATCH_MAP()

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REFERENCES

For more information on MFC dispatch macros and how MFC assigns DISPIDs, see MFC TechNote TN039 in the Visual C++ Documentation.

(c) Microsoft Corporation 1997, All Rights Reserved. Contributions by Kelly Marie Ward, Microsoft Corporation.

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