Q217185: FIX: IRowsetImpl::GetData() Does Not Handle Nulls Correctly

Article: Q217185
Product(s): Microsoft C Compiler
Version(s): 6.0
Operating System(s): 
Keyword(s): kbservicepack kbDatabase kbOLEDB kbProvider kbVC600bug kbVS600sp2 kbVS600SP1 kbVS600sp3
Last Modified: 07-MAY-2001

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, version 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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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

If the Provider correctly updates the length indicator to zero when
DBSTATUS_S_ISNULL is encountered, the following output for the FileName field
should appear in the debug window:

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

If the Provider ignores the length indicator when DBSTATUS_S_ISNULL is
encountered, then ActualSize will report some non-zero value.

Additional query words:

======================================================================
Keywords          : kbservicepack kbDatabase kbOLEDB kbProvider kbVC600bug kbVS600sp2 kbVS600SP1 kbVS600sp3fix kbGrpDSVCDB kbMDACNoSweep 
Technology        : kbVCsearch kbAudDeveloper kbVC600 kbVC32bitSearch
Version           : :6.0
Issue type        : kbbug
Solution Type     : kbfix

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