Q271577: Fix: CManualAccessor Does Not Work With Schema Rowsets
Article: Q271577
Product(s): Microsoft C Compiler
Version(s): 6.0,6.0sp5
Operating System(s):
Keyword(s): kbATL kbDatabase kbDTL kbATL300fix kbGrpDSVCDB kbDSupport kbVS600sp5fix
Last Modified: 07-MAY-2001
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The information in this article applies to:
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 6.0, 6.0sp5
- Microsoft Visual C++, 32-bit Professional Edition, versions 6.0, 6.0sp5
- Microsoft Visual C++, 32-bit Learning Edition, versions 6.0, 6.0sp5
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SYMPTOMS
========
When you use the Active Template Library (ATL) Consumer templates to get schema
rowsets from a data store, it is impossible to use a CManualAccessor class and
base the accessor on the metadata after retrieving the rowset. It is only
possible to set up the accessor before opening the rowset.
The ATL Consumer classes provide specialized template classes for most common
schema rowsets based on a static CAccessor class such as CTables or CColumns.
For this reason the problem described in this article only manifests itself when
you retrieve non-standard schema rowsets and base them on the CManualAccessor
class to avoid creating static accessors.
An example of such schema rowsets are the multi-dimensional schema rowsets
available with the Microsoft OLE DB Provider for OLAP Services, such as
MDSCHEMA_MEMBERS.
CAUSE
=====
The CRestrictions and CSchemaRowset classes did not have a parameter in their
Open method that specified whether an accessor should be bound in the Open
method call. The CSchemaRowset::Open method called the CAccessorRowset::Bind
method automatically, which required the accessor to be set up before the Open
method was called.
RESOLUTION
==========
Install Visual Studio 6.0 Service Pack 5 and use the new parameter available in
the CRestrictions class to indicate that the accessor should not be bound in the
Open call. For additional information, refer to the "More Information" section.
STATUS
======
For additional information about Visual Studio service packs, click the article
numbers below to view the articles in the Microsoft Knowledge Base:
Q194022 INFO: Visual Studio 6.0 Service Packs, What, Where, Why
Q194295 HOWTO: Tell That a Visual Studio Service Pack Is Installed
You can download the latest Visual Studio service pack from the following
Microsoft Web site:
Visual Studio Product Updates
(http://msdn.microsoft.com/vstudio/downloads/updates.asp)
http://msdn.microsoft.com/vstudio/downloads/updates.asp
MORE INFORMATION
================
A new parameter has been added to the CRestrictions::Open and
CSchemaRowset::Open methods called bBind. bBind is the ninth parameter in the
CRestrictions::Open call and the third parameter in the CSchemaRowset::Open
call. Both these parameters have a default true value so existing code that does
not use this parameter works without changes.
The following code sample demonstrates how to use this new parameter with a
CManualAccessor class. The accessor is created dynamically based on the metadata
retrieved from a call to IColumnsInfo::GetColumnInfo. The sample connects to a
Microsoft SQL Server 7.0 or a Microsoft SQL Server 2000 database, and retrieves
column information for the authors table in the pubs database.
To run the code, follow these steps:
1. Create a console application that supports Microsoft Foundation Classes (MFC)
in Visual C++.
2. Include the following header files:
#include <oledberr.h>
#include <atldbcli.h>
#include <atldbsch.h>
3. Paste in the following code to replace the skeleton _tmain code.
SAMPLE CODE
typedef CRestrictions<CManualAccessor, 4, &DBSCHEMA_COLUMNS> CMyColumns;
const int DATA_LENGTH = 128;
struct MYBIND
{
union tagData
{
OLECHAR wszData[DATA_LENGTH];
ULONG ulData;
} data;
DBSTATUS dwStatus;
ULONG ulLength;
MYBIND()
{
memset(this,0,sizeof(MYBIND));
}
};
int _tmain(int argc, TCHAR* argv[], TCHAR* envp[])
{
int nRetCode = 0;
// initialize MFC and print and error on failure
if (!AfxWinInit(::GetModuleHandle(NULL), NULL, ::GetCommandLine(), 0))
{
// TODO: change error code to suit your needs
cerr << _T("Fatal Error: MFC initialization failed") << endl;
nRetCode = 1;
}
else
{
CDataSource DataSource;
CSession Session;
HRESULT hr;
hr = CoInitialize(NULL);
hr = DataSource.OpenFromInitializationString(
L"Provider=SQLOLEDB.1;Persist Security Info=False;User ID=sa;Initial Catalog=Pubs;Data Source=(local)");
hr = Session.Open(DataSource);
CMyColumns columns;
//Uncomment the following line to see the pre-SP5 behavior, comment out line after that
//hr = columns.Open(Session, "pubs", NULL, "authors", NULL, NULL, NULL, NULL);
hr = columns.Open(Session, "pubs", NULL, "authors", NULL, NULL, NULL, NULL, false);
CComPtr<IColumnsInfo> pColumnsInfo;
hr = columns.m_spRowset->QueryInterface(IID_IColumnsInfo, (void**) &pColumnsInfo);
DBORDINAL ColCount;
DBCOLUMNINFO* pColInfo;
OLECHAR* wszInfo;
hr = pColumnsInfo->GetColumnInfo(&ColCount,&pColInfo, &wszInfo);
MYBIND* pData = new MYBIND[ColCount];
hr = columns.CreateAccessor(ColCount,pData, ColCount * sizeof(MYBIND));
int i;
for (i = 0; i < ColCount; i++)
{
//We are only binding to two different data types, UI4 and WSTR
//A more elaborate scheme can be implemented here
if (pColInfo[i].wType == DBTYPE_UI4 || pColInfo[i].wType == DBTYPE_UI2)
{
columns.AddBindEntry(i+1,
DBTYPE_UI4,
4,
&(pData[i].data.ulData),
&(pData[i].ulLength),
&(pData[i].dwStatus));
}
else
{
columns.AddBindEntry(i+1,
DBTYPE_WSTR,
DATA_LENGTH,
&(pData[i].data.wszData),
&(pData[i].ulLength),
&(pData[i].dwStatus));
}
TRACE("%i) %S ", i, pColInfo[i].pwszName);
}
TRACE("\n");
hr = columns.Bind();
while (SUCCEEDED(hr = columns.MoveNext()) && hr != DB_S_ENDOFROWSET )
{
for(i = 0; i < ColCount; i++)
{
if (pData[i].dwStatus != DBSTATUS_S_ISNULL)
{
if (pColInfo[i].wType == DBTYPE_UI4 || pColInfo[i].wType == DBTYPE_UI2)
{
TRACE("%i) %i ", i, pData[i].data.ulData);
}
else
{
TRACE("%i) %S ", i, pData[i].data.wszData);
}
}
}
TRACE("\n");
}
delete [] pData;
CoTaskMemFree(pColInfo);
CoTaskMemFree(wszInfo);
}
return nRetCode;
}
4. Modify the connection string to reflect the name of your SQL Server server.
Additional query words: DB_S_ERRORSOCCURRED DB_E_ERRORSOCCURRED
======================================================================
Keywords : kbATL kbDatabase kbDTL kbATL300fix kbGrpDSVCDB kbDSupport kbVS600sp5fix
Technology : kbVCsearch kbAudDeveloper kbVC600 kbVC32bitSearch kbVC600SP5
Version : :6.0,6.0sp5
Issue type : kbbug
Solution Type : kbfix
=============================================================================
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