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;