Fluent interface with inheritance in Delphi

Question

I have following fluent interface declaration and class that implements that interface:

type
  IDocWriter = interface
    ['{8CB5799A-14B1-4287-92FD-41561B237560}']
    function Open: IDocWriter;
    function Close: IDocWriter;
    function Add(const s: string): IDocWriter;
    function SaveToStream(Stream: TStream): IDocWriter;
  end;

  TDocWriter = class(TInterfacedObject, IDocWriter)
  public
    function Open: IDocWriter;
    function Close: IDocWriter;
    function Add(const s: string): IDocWriter;
    function SaveToStream(Stream: TStream): IDocWriter;
  end;

{ TDocWriter }

function TDocWriter.Open: IDocWriter;
begin
  Result := Self;
  // DoOpen
end;

function TDocWriter.Close: IDocWriter;
begin
  Result := Self;
  // DoClose
end;

function TDocWriter.Add(const s: string): IDocWriter;
begin
  Result := Self;
  // DoAdd
end;

function TDocWriter.SaveToStream(Stream: TStream): IDocWriter;
begin
  Result := Self;
  // DoSaveToStream
end;

And I can use above code like this:

var
  Stream: TStream;
  ...
  TDocWriter.Create
    .Open
    .Add('abc')
    .Close
    .SaveToStream(Stream);

I have to extend above interface by adding SaveToString function.

I don't want to add that method to original IDocWriter interface because it is not valid method for all interface implementations. So I have done following

type
  IStrDocWriter = interface(IDocWriter)
    ['{177A0D1A-156A-4606-B594-E6D20818CE51}']
    function SaveToString: string;
  end;

  TStrDocWriter = class(TDocWriter, IStrDocWriter)
  public
    function SaveToString: string;
  end;

{ TStrDocWriter }

function TStrDocWriter.SaveToString: string;
begin
  Result := 'DoSaveToString';
end;

In order to use IStrDocWriter interface I have to write code

var
  Writer: IDocWriter;
  s: string;

  Writer := TStrDocWriter.Create
    .Open
    .Add('abc')
    .Close;
  s := (Writer as IStrDocWriter).SaveToString;

But I would like to be able to use it without the need to declare Writer variable, something like following code (which, of course, cannot be compiled)

  s := TStrDocWriter.Create
    .Open
    .Add('abc')
    .Close
    .SaveToString;   // Undeclared identifier SaveToString

Is there any way to achieve that?

Any kind of changes to above interfaces and classes are acceptable (except, obviously, merging those two interfaces into one).

Answer 1

You can write it like this:

s := (TStrDocWriter.Create
    .Open
    .Add('abc')
    .Close as IStrDocWriter)
    .SaveToString; 

No, not very nice is it. These two idioms, fluent interfaces and inheritance, simply do not mix.