I've come across a bug in BO Mapper when adding a FieldChanging event to a field that allows nulls. 
The following code was generated by the mapper. The "If value..." comparison always results to false, 
since the value was set equal to the e.FieldValue in the prior line, resulting in the item always 
being set to DBNull.  

Set(ByVal value As System.Int32)
   Dim e As New TestBOFieldChangingEventArgs(TestBOFieldNames.ta_tst_id, value)
   Me.OnFieldPropertyChanging(Me, e)
   value = CType(e.FieldValue, System.Int32)
   If value <> CType(e.FieldValue, System.Int32) Then <---- Always False

        Me.CurrentRow.Item("ta_tst_id") = value
   Else
        Me.CurrentRow.Item("ta_tst_id") = DBNull.Value
   End IfEnd Set

Since I've configured the item to be Return Alternate on Null / Set Null on Alternate to 0 
I believe the if statement should be "If value <> 0 Then" as it does when adding a FieldChanged event.

The following code was generated by the mapper for a field where Alternate on null was set to 0 with
both FieldChanged and FieldChanging events. The FieldChanged event code uses "If value <> 0" while the
FieldChanging event code is using "If value <> Ctyp..."

Set(ByVal value As System.Int32)
   Dim e As New TestBOFieldChangingEventArgs(TestBOFieldNames.ta_tst_id, value)
   Dim llRaiseEvent As Boolean = False
   Dim loRow As DataRow = Me.CurrentRow
   Me.OnFieldPropertyChanging(Me, e)
       value = CType(e.FieldValue, System.Int32)
       If value <> 0 Then
           If Not loRow.Item("ta_tst_id").Equals(value) Then llRaiseEvent = True
       Else
           If Not loRow.Item("ta_tst_id").Equals(DBNull.Value) Then llRaiseEvent = True
       End If
       If value <> CType(e.FieldValue, System.Int32) Then
           loRow.Item("ta_tst_id") = value
       Else
           loRow.Item("ta_tst_id") = DBNull.Value
       End If
       If llRaiseEvent Then Me.OnFieldPropertyChanged(Me, New TestBOFieldChangedEventArgs(TestBOFieldNames.ta_tst_id))
End Set