Q138767: FIX: CreateRelation Method Fails with Non-UNICODE Build
Article: Q138767
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s):
Keyword(s): kbcode kbDAOsearch kbDatabase kbMFC kbVC kbVC410fix
Last Modified: 30-JUL-2001
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The information in this article applies to:
- The Microsoft Foundation Classes (MFC), included with:
- *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, version 4.0
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SYMPTOMS
========
The CDaoDatabase::CreateRelation() method may fail if an application is built as
a non-UNICODE application. It is possible that a messagebox will appear that
states:
Invalid field name '<some text>' in definition of index or relationship
CAUSE
=====
A bug exists in the CDaoDatabase::CreateRelation() function. In
\Msdev\Mfc\Src\Daocore.cpp, you can see the following code in the
CreateRelation() function:
COleVariant var;
var = relinfo.m_pFieldInfos[nIndex].m_strForeignName;
DAO_CHECK(pDAOField->put_ForeignName(V_BSTR(&var)));
This code is incorrect. Here is the correct code:
COleVariant var(relinfo.m_pFieldInfos[nIndex].m_strForeignName,
VT_BSTRT);
DAO_CHECK(pDAOField->put_ForeignName(V_BSTR(&var)));
The fixed code ensures that the BSTR for the variant will be in ANSI format for
non-UNICODE versions of an application and will be in UNICODE format for UNICODE
versions of an application.
RESOLUTION
==========
Perform the following steps:
1. Derive a new class from CDaoDatabase.
2. Add the function prototypes for the two CreateRelation() functions to your
new class declaration.
3. Copy the two CreateRelation() functions from Daocore.cpp to the
CreateRelation() functions of your new CDaoDatabase-derived class.
4. Change the following code in your new CreateRelation(CDaoRelationInfo&
relinfo) method from:
COleVariant var;
var = relinfo.m_pFieldInfos[nIndex].m_strForeignName;
DAO_CHECK(pDAOField->put_ForeignName(V_BSTR(&var)));
to:
COleVariant var(relinfo.m_pFieldInfos[nIndex].m_strForeignName,
VT_BSTRT);
DAO_CHECK(pDAOField->put_ForeignName(V_BSTR(&var)));
5. Declare a local VARIANT and initialize it in the overridden function as
follows:
VARIANT var;
var.vt = VT_ERROR;
var.scode = DISP_E_PARAMNOTFOUND;
This is necessary because the _afxOptionalVariant used in the original
CreateRelation() function is defined in DAOCORE.CPP,
6. Replace instances of the _afxOptionalVariant variable with var.
Instead of using the CDaoDatabase class, you will use your new CDaoDatabase-
derived class. The sample code in this article demonstrates what the new code
might look like.
STATUS
======
Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug was corrected in Visual C++ 4.1.
MORE INFORMATION
================
Sample Code to Demonstrate Fixed Code
-------------------------------------
This demonstrates how you will use your new CDaoDatabase-derived class; in this
case called CMyDatabase.
CMyDatabase db;
db.Open("f:\\test\\relation\\relate.mdb");
db.CreateRelation(_T("mytest"), _T("Table1"), _T("Table2"),
dbRelationUnique|dbRelationUpdateCascade,_T("field1"),
_T("field1"));
db.Close();
Here is the declaration of the new class:
class CMyDatabase:public CDaoDatabase
{
public:
void CreateRelation(LPCTSTR lpszName, LPCTSTR lpszTable,
LPCTSTR lpszForeignTable, long lAttributes,
LPCTSTR lpszField,
LPCTSTR lpszForeignField);
void CreateRelation(CDaoRelationInfo& relinfo);
};
Here is the implementation of the new class:
void CMyDatabase::CreateRelation(LPCTSTR lpszName, LPCTSTR lpszTable,
LPCTSTR lpszForeignTable, long lAttributes, LPCTSTR lpszField,
LPCTSTR lpszForeignField)
{
ASSERT_VALID(this);
CDaoRelationInfo relinfo;
CDaoRelationFieldInfo fieldinfo;
relinfo.m_strName = lpszName;
relinfo.m_strTable = lpszTable;
relinfo.m_strForeignTable = lpszForeignTable;
relinfo.m_lAttributes = lAttributes;
relinfo.m_nFields = 1;
relinfo.m_pFieldInfos = &fieldinfo;
relinfo.m_pFieldInfos->m_strName = lpszField;
relinfo.m_pFieldInfos->m_strForeignName = lpszForeignField;
CreateRelation(relinfo);
}
void CMyDatabase::CreateRelation(CDaoRelationInfo& relinfo)
{
ASSERT_VALID(this);
ASSERT(IsOpen());
ASSERT(relinfo.m_nFields > 0);
VARIANT var;
var.vt = VT_ERROR;
var.scode = DISP_E_PARAMNOTFOUND;
// Initialize relations collection so that relation can be
// appended later
if (m_pDAORelations == NULL)
InitRelationsCollection();
DAORelation* pDAORelation = NULL;
DAOFields* pDAOFields = NULL;
DAOField* pDAOField = NULL;
// Create the relation
DAO_CHECK(m_pDAODatabase->CreateRelation(
COleVariant(relinfo.m_strName, VT_BSTRT),
COleVariant(relinfo.m_strTable, VT_BSTRT),
COleVariant(relinfo.m_strForeignTable, VT_BSTRT),
COleVariant(relinfo.m_lAttributes), &pDAORelation));
TRY
{
// Get the fields collection for later append of created field
DAO_CHECK(pDAORelation->get_Fields(&pDAOFields));
// Create field(s) and set the name and foreign name
for (int nIndex = 0; nIndex < relinfo.m_nFields; nIndex++)
{
// note that we are using 'var' in the following function
// instead of _afxOptionalVar declared in DAOCORE.CPP
DAO_CHECK(pDAORelation->CreateField(
COleVariant(relinfo.m_pFieldInfos[nIndex].m_strName,
VT_BSTRT), var, var, &pDAOField));
COleVariant var(relinfo.m_pFieldInfos[nIndex].m_strForeignName,
VT_BSTRT);
DAO_CHECK(pDAOField->put_ForeignName(V_BSTR(&var)));
// Append the field to relation fields collection and
// release
DAO_CHECK(pDAOFields->Append(pDAOField));
pDAOField->Release();
}
DAO_CHECK(m_pDAORelations->Append(pDAORelation));
}
CATCH_ALL(e)
{
// Clean up before throw
if (pDAOField != NULL)
pDAOField->Release();
if (pDAOFields != NULL)
pDAOFields->Release();
pDAORelation->Release();
THROW_LAST();
}
END_CATCH_ALL
// Clean up
if (pDAOField != NULL)
pDAOField->Release();
pDAOFields->Release();
pDAORelation->Release();
}
Additional query words: kbVC400bug
======================================================================
Keywords : kbcode kbDAOsearch kbDatabase kbMFC kbVC kbVC410fix
Technology : kbAudDeveloper kbMFC
Version : winnt:
Issue type : kbbug
Solution Type : kbfix
=============================================================================
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