FIX: CManualAccessor Does Not Work With Schema Rowsets (271577)


The information in this article applies to:

  • Microsoft Visual C++, 32-bit Enterprise Edition 6.0
  • Microsoft Visual C++, 32-bit Professional Edition 6.0
  • Microsoft Visual C++, 32-bit Learning Edition 6.0
This article was previously published under Q271577

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 following article numbers to view the articles in the Microsoft Knowledge Base:

194022 INFO: Visual Studio 6.0 Service Packs, What, Where, Why

194295 HOWTO: Tell That a Visual Studio Service Pack Is Installed

To download the latest Visual Studio service pack, visit the following Microsoft Web site:

http://msdn.microsoft.com/vstudio/downloads/updates/

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



    Note You must change the User ID <username> value and the password =<strong password> value to the correct values before you run this code. Make sure that User ID has the appropriate permissions to perform this operation on the database. 
    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=<username>;Password=<strong password>;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.
Modification Type:MinorLast Reviewed:12/12/2005
Keywords:kbATL300fix kbbug kbDatabase kbDTL kbfix kbVS600sp5fix KB271577 kbAudDeveloper
©2004 Microsoft Corporation. All rights reserved.