Question

Can i typecast a base interface to derived interface.

How do I make like Delphi

Delphi generated code / Delphi 7 :

function TXMLBook.Get_Id: IXMLId;
begin
   Result := ChildNodes['id'] as IXMLId;
end;

ChildNodes[‘id’] is base type IXMLNode

IXMLId is derived type from IXMLNode

and my sample code is :

type
IBase = interface(IInterface)
['{5CEA3668-36CB-444C-8D3E-06644070A2B4}']
  function Test:AnsiString;
end;

TBase =class(TInterfacedObject,IBase)
  function Test:AnsiString;
end;

IDerived =interface (IBase)
['{8CDB2982-43B0-4636-B6F5-2561C0D419A9}']
  function Test2: AnsiString;
end;

TDerived=class (TBase,IDerived)
  function Test:AnsiString;
  function Test2:AnsiString;
end;

{ TBase }

function TBase.Test: AnsiString;
begin
   Result:='Base Test';
end;

{ TDerived }

function TDerived.Test: AnsiString;
begin
  Result:='Derived Test';
end;

function TDerived.Test2: AnsiString;
begin
   Result:='Derived Test2';
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  b:IBase;
  d: IDerived;
begin
  b:=TBase.Create;
  d:=b as IDerived; // Error : Interface not supported 

  ShowMessage(b.Test);
end;

Answer 1

Your TBase class currently does not implement IDerived, it only implements IBase.
Your TDerived class implements both IBase (by inheriting from TBase) and IDerived (explicitly).

You should either

1. Change your TBase declaration to

   TBase =class(TInterfacedObject,IBase, IDerived)

   and implement the IDerived methods in TBase.

2. or change b:=TBase.Create; to b:=TDerived.Create;