FIX: IRowsetImpl::GetData() Does Not Handle Nulls Correctly

• Microsoft Visual C++, 32-bit Editions, version 6.0

SYMPTOMS

When an OLE DB Provider returns a DBSTATUS of DBSTATUS_S_ISNULL for a text field created using the ATL OLE DB Provider templates then the length of the field returned to the OLE DB consumer is not explicitly set to zero. This behavior follows the OLE DB specification, but can cause problems if an OLE DB consumer application utilizes the field length without checking the DBSTATUS of the column first. For example, a consumer application may assume that a non-zero field length means that the field is not NULL which is a common assumption when using text fields.

CAUSE

IRowsetImpl::GetData does not explicitly set the length for the column to zero if the DBSTATUS of the column is set to DBSTATUS_S_ISNULL.

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 Studio 6.0 Service Pack 3. For more information about Visual Studio service packs, please see the following articles in the Microsoft Knowledge Base:

Q194022 INFO: Visual Studio 6.0 Service Packs, What, Where, Why

Q194295 HOWTO: Tell That Visual Studio 6.0 Service Packs Are Installed

MORE INFORMATION

When implementing an CRowsetImpl derived class using the ATL OLE DB Provider template class, you can override the GetDBStatus function to provide DBSTATUS information for each field returned by the OLE DB Provider. If you return DBSTATUS_S_ISNULL from the GetDBStatus function for a particular field, this indicates to the OLE DB consumer that the particular field is NULL.

Internally, IRowsetImpl::GetData checks the DBSTATUS value you set for each field when returning the data to the consumer. If you set DBSTATUS to DBSTATUS_S_ISNULL, IRowsetImpl::GetData does not update the length indicator for a text field to 0. IRowsetImpl::GetData returns an unmodified length indicator to the client if you set the DBSTATUS of a field to DBSTATUS_S_ISNULL.

Steps to reproduce

1. Create a new ATL OLE DB Provider named TESTPROV using the ATL OLE DB Provider Wizard.



2. Add a GetDBStatus method to the CTESTPROVRowset class as shown in the following code sample:




   class CTESTPROVRowset : public CRowsetImpl< CTESTPROVRowset,
   CTESTPROVWindowsFile, CTESTPROVCommand>                               
   {
      public:
         HRESULT Execute(DBPARAMS * pParams, LONG* pcRowsAffected)
         {
             // Etc....
         }
 //Add the following method      
         DBSTATUS GetDBStatus( CSimpleRow* pRow , ATLCOLUMNINFO* pColInfo )
         {
            // Force column 4 (FileName) to null for every single row.
            if ( 4 == pColInfo->iOrdinal )
               return DBSTATUS_S_ISNULL;
            return DBSTATUS_S_OK;
         }
   }; 

3. Build the project to create and register the TESTPROV OLE DB Provider DLL.



4. Test the Provider using the following VB client code:

   ' The following code requires a reference to Microsoft ActiveX Data
   ' Objects library in your VB project in order to compile correctly.
   Sub CheckTESTPROV()
   Dim rs As New ADODB.recordset
   Dim f As ADODB.Field
   Dim strFieldStatus As String
      rs.Open "c:\*.*", "Provider=TESTPROV OLE DB Provider;"
      For Each f In rs.fields
         If IsNull(f.Value) Then
            strFieldStatus = "DBSTATUS_S_ISNULL"
         Else
            strFieldStatus = "DBSTATUS_S_OK"
         End If
         Debug.Print "[" & f.Name & "]"
         Debug.Print "  Field Status = " & strFieldStatus
         Debug.Print "  DefinedSize  = " & f.DefinedSize
         Debug.Print "  ActualSize   = " & f.ActualSize
      Next f
   End Sub 

[FileName]
  Field Status = DBSTATUS_S_ISNULL
  DefinedSize  = 260
  ActualSize   = 0 

Additional query words:

Keywords : kbservicepack kbDatabase kbOLEDB kbProvider kbVC600bug kbVS600sp2 kbVS600SP1 kbVS600sp3fix kbGrpVCDB
Version : winnt:6.0
Platform : winnt
Issue type : kbbug