FIX: Setting Default Value Using CreateField Gives Wrong Value
ID: Q171316
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The information in this article applies to:
-
Microsoft Visual C++, 32-bit Editions, version 5.0sp1
SYMPTOMS
An MFC DAO application built using Visual C++ version 4.2x and linked to
MFC42.DLL incorrectly sets the default value for a field. For example, if
CDaoTableDef::CreateField() is called with the
CDaoFieldInfo::m_strDefaultValue set to "default value", you will find that
a "d" is added as the default value instead of the whole string.
For sample code that demonstrates the problem, see the MORE INFORMATION
section below.
CAUSE
There is a bug in version 4.21.7160 of MFC42.DLL (this is the MFC DLL that
comes with Visual C++ 5.0 Service Pack 1). In the AfxSetDefaultValue()
function located in DAOCORE.CPP, you can see the following code:
.
.
.
else
{
// Call DAO 3.0 method
// put_DefaultValue takes BSTR param not VARIANT
HRESULT (STDMETHODCALLTYPE DAOField::*pMethod)(BSTR) =
(HRESULT (STDMETHODCALLTYPE
DAOField::*)(BSTR))pDAOField->put_DefaultValue;
BSTR bstr = strDefaultValue.AllocSysString();
DAO_CHECK((pDAOField->*pMethod)(bstr));
SysFreeString(bstr);
}
The bstr value needs to be created as an ANSI BSTR, which DAO expects in a
non-UNICODE application rather than a UNICODE BSTR that the code is
creating.
RESOLUTION
You can work around this problem by deriving a new class from CDaoTableDef
that defines a new CreateField() function. If you use this code in your
Visual C++ 4.2x application and later decide to build your application with
Visual C++ 5.0 or greater (which uses DAO 3.5 and doesn't experience the
bug), you don't want to use new class. The code demonstrates one way that
you can check the _MFC_VER constant to determine whether the code is
needed:
.H File
#if _MFC_VER <0x0421
class CNewDaoTableDef: public CDaoTableDef
{
public:
CNewDaoTableDef(CDaoDatabase * pDatabase):CDaoTableDef(pDatabase){};
void CreateField(CDaoFieldInfo& fieldinfo);
void SetFieldInfo(DAOField* pDAOField, CDaoFieldInfo& fieldinfo);
};
#endif // _MFC_VER <0x421
.CPP File
#if _MFC_VER <0x0421
void CNewDaoTableDef::CreateField(CDaoFieldInfo& fieldinfo)
{
ASSERT_VALID(this);
DAOField* pDAOField;
// Create the DAO field object (setting basic properties)
DAO_CHECK(m_pDAOTableDef->CreateField(
COleVariant(fieldinfo.m_strName, VT_BSTRT),
COleVariant(fieldinfo.m_nType),
COleVariant(fieldinfo.m_lSize), &pDAOField));
TRY
{
SetFieldInfo(pDAOField, fieldinfo);
// Append the field object to the fields collection
if (m_pDAOFields == NULL)
InitFieldsCollection();
DAO_CHECK(m_pDAOFields->Append(pDAOField));
}
CATCH_ALL(e)
{
pDAOField->Release();
THROW_LAST();
}
END_CATCH_ALL
pDAOField->Release();
}
void CNewDaoTableDef::SetFieldInfo(DAOField* pDAOField, CDaoFieldInfo&
fieldinfo)
{
// Assumes name, type and size set on direct DAO CreateField call
ASSERT(pDAOField != NULL);
if (fieldinfo.m_lAttributes != 0)
DAO_CHECK(pDAOField->put_Attributes(fieldinfo.m_lAttributes));
if (fieldinfo.m_nOrdinalPosition != 0)
{
DAO_CHECK(pDAOField
->put_OrdinalPosition(fieldinfo.m_nOrdinalPosition));
}
if (fieldinfo.m_bRequired)
DAO_CHECK(pDAOField->put_Required(AFX_DAO_TRUE));
if (fieldinfo.m_bAllowZeroLength)
DAO_CHECK(pDAOField->put_AllowZeroLength(AFX_DAO_TRUE));
if (!fieldinfo.m_strForeignName.IsEmpty())
{
COleVariant var(fieldinfo.m_strForeignName, VT_BSTRT);
DAO_CHECK(pDAOField->put_ForeignName(V_BSTR(&var)));
}
if (!fieldinfo.m_strValidationRule.IsEmpty())
{
COleVariant var(fieldinfo.m_strValidationRule, VT_BSTRT);
DAO_CHECK(pDAOField->put_ValidationRule(V_BSTR(&var)));
}
if (!fieldinfo.m_strValidationText.IsEmpty())
{
COleVariant var(fieldinfo.m_strValidationText, VT_BSTRT);
DAO_CHECK(pDAOField->put_ValidationText(V_BSTR(&var)));
}
if (!fieldinfo.m_strDefaultValue.IsEmpty())
{
COleVariant var(fieldinfo.m_strDefaultValue, VT_BSTRT);
DAO_CHECK(pDAOField->put_DefaultValue(V_BSTR(&var)));
}
}
#endif // _MFC_VER <0x0421
STATUS
Microsoft has confirmed this to be a bug in the Microsoft products listed
at the beginning of this article. This bug has been fixed in Visual Studio
97 Service Pack 3.
For more information, please see the following article in the Microsoft
Knowledge Base:
Q170365 INFO: Visual Studio 97 Service Packs - What, Where, and Why
MORE INFORMATION
Sample Code
This sample code fails if the application is built with Visual C++ 4.2x and
you use the newer MFC42.DLL (the one with Visual C++ 5.0 service pack 1).
CDaoDatabase db;
db.Open(_T("db.mdb"));
CDaoTableDef td(&db);
// Replace the line above with the following code to fix the problem:
// #if _MFC_VER < 0x0421
// CNewDaoTableDef td(&db); // need to use new class w/ VC++ 4.2x
// #else
// CDaoTableDef td(&db); // building with VC++ 5.0 the bug doesn't
// exist #endif
td.Create(_T("FieldTest"));
CDaoFieldInfo fi;
fi.m_strName = _T("testfield");
fi.m_nType = dbText;
fi.m_lSize = 50;
fi.m_lAttributes = dbFixedField;
fi.m_nOrdinalPosition = 0;
fi.m_bRequired = FALSE;
fi.m_bAllowZeroLength = TRUE;
fi.m_lCollatingOrder = dbSortGeneral ;
fi.m_strDefaultValue = _T("default value");
td.CreateField(fi);
td.Append();
CDaoRecordset rs(&db);
rs.Open(dbOpenDynaset, _T("Select * from FieldTest3"));
rs.AddNew();
rs.Update();
rs.Close();
db.Close();
Additional query words:
4.2b
Keywords : kbDAO kbDatabase kbMFC kbVC kbVS97sp2fix
Version : 5.0sp1
Platform : NT WINDOWS
Issue type : kbbug