function TMyClass.CreateCalcField(const AFieldName: string;
 AFieldClass: TFieldClass; ASize: Word): TField;
begin
 Result := FDataSet.FindField(AFieldName); // Field may already exists!
 if Result <> nil then
  Exit;
 if AFieldClass = nil then
 begin
  DBErrorFmt(SUnknownFieldType, [AFieldName]);
 end;
 Result := FieldClass.Create(Owner);
 with Result do
 try
  FieldName := AFieldName;
  if (Result is TStringField) or (Result is TBCDField) or
  (Result is TBlobField) or (Result is TBytesField) or
  (Result is TVarBytesField) then
  begin
  Size := ASize;
  end;
  Calculated := True;
  DataSet := FDataset;
  Name := FDataSet.Name + AFieldName;
 except
  Free; // We must release allocated memory on error!
  raise;
 end;
end;