FIX: IRowsetImpl::GetData() Does Not Handle Nulls Correctly (217185)

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.