在Delphi中具有继承性的流畅接口

Question

我有以下流利的接口声明和实现该接口的类：

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;

而且我可以像上面这样使用上面的代码：

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

我必须通过添加SaveToString功能来扩展接口。

我不想将该方法添加到原始IDocWriter接口，因为它不是所有接口实现的有效方法。所以我做了以下

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;

为了使用IStrDocWriter接口，我必须编写代码

var
  Writer: IDocWriter;
  s: string;

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

但是我希望能够在不需要声明Writer变量的情况下使用它，就像下面的代码（当然，不能编译）

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

有什么办法可以实现？

可以对上述接口和类进行任何形式的更改（显然，将这​​两个接口合并为一个除外）。