Q142223: INFO: Variant Handling in an MFC Server w/ VB 4.0 as Its Client

Article: Q142223
Product(s): Microsoft C Compiler
Version(s): WINDOWS:4.0; winnt:2.0,2.1,2.2,4.0
Operating System(s): 
Keyword(s): kbcode kbActiveX kbAutomation kbCOMt kbMFC kbVBp400 kbVC200 kbVC400 kbGrpDSMFCATL
Last Modified: 06-MAY-2001

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The information in this article applies to:

- Microsoft Visual Basic Standard Edition for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition for Windows, version 4.0 
- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0 
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SUMMARY
=======

It is possible for Visual Basic 4.0 to pass parameters to automation methods
either by value (ByVal) or by reference (ByRef). For a method that expects a
non-VARIANT parameter passed by value, MFC's implementation of IDispatch will
coerce a parameter passed by reference to a value. However, this is not the case
with an automation method expecting a VARIANT parameter. The MFC framework
cannot coerce a VARIANT parameter because it does not know what type is expected
by the method. This can cause problems in a method that expects a VARIANT
parameter to be passed by value. If the parameter is passed by reference and the
VARIANT is referenced directly, the value obtained from the variant will be
incorrect. Hence, an automation method that expects a VARIANT to be passed by
value should check whether it was passed a reference and coerce the VARIANT if
necessary. This article illustrates how you could implement this.

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

Consider the following method named Add exposed by a MFC automation server with
a ProgID TestVar.Document:

     LPDISPATCH Add(const VARIANT FAR& varTest);

Given the following code in Visual Basic, it may pass the parameter either by
reference or by value:

     Dim doc As Object
     Set doc = CreateObject("TestVar.Document")

     Dim docDispatch As Object
     Dim varParam As Variant
     varParam = 2

Visual Basic will pass the parameter by reference in these cases:

     Set docDispatch = doc.Add(varParam)
     doc.Add varParam

Visual Basic will pass the parameter by value in these cases:

     Set docDispatch = doc.Add((varParam))
     doc.Add (varParam)
     Set docDispatch = doc.Add(2)
     doc.Add 2

From these examples, you may notice that Visual Basic will pass all variables by
reference unless the () operator is used to indicate that the variable should be
passed by value. Moreover, Visual Basic will pass all constants by value.

Because of the possibility of the parameter being passed by reference, it is
necessary to coerce the parameter to a value before using it in the automation
server. Making a copy of the VARIANT parameter using the VariantCopyInd function
will perform the necessary indirection, if the source VARIANT parameter passed
is specified to be VT_BYREF. The following code will properly handle a VARIANT
passed either by reference or by value.

  LPDISPATCH CTestVarDoc::Add(const VARIANT FAR& varTest)
  {
      HRESULT hr;
      VARIANT var;
      VariantInit(&var);

      hr = VariantCopyInd(&var, (LPVARIANT)&varTest);

      if (FAILED(hr))
         return NULL;

      // Now use var instead of varTest
      ...

  }

Additional query words: kbinf

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Keywords          : kbcode kbActiveX kbAutomation kbCOMt kbMFC kbVBp400 kbVC200 kbVC400 kbGrpDSMFCATL 
Technology        : kbVBSearch kbAudDeveloper kbPTNotAssigned kbMFC kbZNotKeyword2
Version           : WINDOWS:4.0; winnt:2.0,2.1,2.2,4.0
Issue type        : kbinfo

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