PRB: Visual Basic String (BSTR) in Variant Field in ADO Recordset Cannot Be Persisted to ADTG

If you create an ActiveX Data Objects (ADO) or ADOR recordset with a field of type adVariant, and populate that field with a Microsoft Visual Basic string value, then call the Recordset's Save method, specifying ADTG as the persistence format, the Save method appears to work with MDAC 2.1, but fails under MDAC version 2.5 or later with the following error message:

The ADTG persistence format available for ADO Recordsets does not support the following Variant field types: VT_RECORD, VT_ILLEGAL, VT_VARIANT, VT_UNKNOWN, VT_BSTR, and VT_DISPATCH. These variant subtypes are explicitly disallowed by the persistence provider for ADTG. The XML persistence format supports a slightly different set of types.

Because Visual Basic strings cannot be persisted to ADTG format in a field of ADO datatype adVariant, a string datatype such as adChar or adVarchar must be used for Visual Basic string fields.

This behavior is by design and is now more clearly documented in the MDAC 2.5 documentation than in previous MDAC documentation versions.

Under MDAC 2.1, the attempt to persist a Visual Basic string in ADTG format in an adVariant field did not generate an error message. However the data that MDAC 2.1 saved in this case was not, in fact, valid. Therefore, the error message that is raised by MDAC 2.5 or later indicates improved error-handling in the persistence provider, not a change in behavior.

Steps to Reproduce Behavior

1. Paste the following code into an event procedure in a Visual Basic project, with a reference to the ADOR library. Note that field "C" is of type adVariant and is being filled with Visual Basic String values.

   Dim intI As Integer
   Dim RS As ADOR.Recordset
   Dim strTempFile As String
   Dim varV As Variant
       
   Set RS = New ADOR.Recordset
   With RS
       .Fields.Append "A", adInteger
       .Fields.Append "B", adBSTR
       .Fields.Append "C", adVariant, , adFldIsNullable
       .Fields.Append "D", adSingle
       .Fields.Append "E", adSmallInt
       .Fields.Append "F", adTinyInt
       .Fields.Append "G", adBigInt
       .Fields.Append "H", adBoolean
       .Fields.Append "I", adDBTimeStamp
       .Open
       .Fields("A").Properties("Optimize") = True
   End With
           
   For intI = 1 To 100
       RS.AddNew
       varV = "abcde"
       With RS
           .Fields("A") = intI
           .Fields("B") = "abcde"
           .Fields("C") = varV
           .Fields("D") = 123.456
           .Fields("E") = 10000
           .Fields("F") = 100
           .Fields("G") = 1000000
           .Fields("H") = True
           .Fields("I") = Now
       End With
       RS.Update
   Next intI
   
   strTempFile = "c:\ADOPersistence.TMP"
   On Error Resume Next
   Kill strTempFile
   On Error GoTo 0
   RS.Save strTempFile, adPersistADTG
   RS.Close
   					

2. Test the code with MDAC 2.1, and note that it appears to succeed.
3. Test the code with MDAC 2.5 or later, and note that it fails with the conversion error message noted in the "Symptoms" section.