Different behavior of interface between Delphi 7 and Delphi XE and higher

Question

I am having a strange problem of using interface in different versions of Delphi. The following minimized code compiles and runs as expected in Delphi XE and higher but not in Delphi 7. Specificaly, it seems when compiling in Delphi 7, the function TForm1.Load: IMoleculeSubject; does not returns the correct result, i.e., the correct reference to the newly created instance. Could you help to comment about the reason and possible workaround? Many thanks!

uInterface.pas

    unit uInterface;
    
    interface
    
    type
    
      IMoleculeSubject = interface
      ['{BEB4425A-186C-45DF-9DCE-C7175DB0CA90}']
      end;
    
      TMoleculeSubject = class(TInterfacedObject, IMoleculeSubject)
      end;
    
    implementation
    
    end.
    

uBusiness.pas

    unit uBusiness;
    
    interface
    
    uses
      uInterface;
    
    type
    
      TMoleculeDecorator = class(TMoleculeSubject) 
      private
        FID: Integer;
      public           
        property ID: Integer read FID;
        constructor Create;
      end;
    
    implementation
    
    { TMoleculeDecorator }
    
    constructor TMoleculeDecorator.Create;
    begin
      inherited Create;
    
      FID := Random(100);
    end;
    
    end.

Unit1.pas

    unit Unit1;
    
    interface
    
    uses
      uInterface, uBusiness,
    
      Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, 
      Forms, Dialogs;
    
    type
      TForm1 = class(TForm)
        procedure FormCreate(Sender: TObject);
      private
        function Load: IMoleculeSubject;
      public
      end;
    
    var
      Form1: TForm1;
    
    implementation
    
    {$R *.dfm}
    
    procedure TForm1.FormCreate(Sender: TObject);
    var
      MolSubject: IMoleculeSubject;
    begin
      MolSubject := Load;
    
              // The down-cast is to show the returned result is wrong in Delphi 7!
      Caption := IntToStr(TMoleculeDecorator(MolSubject).ID);
    end;
    
    function TForm1.Load: IMoleculeSubject;   
    var
      MolSubject: IMoleculeSubject;
    begin
      MolSubject := TMoleculeDecorator.Create;
      Result := MolSubject;
    end;
    
    end.
    
    

Answer 1

The Load function works perfectly well in all versions of Delphi. The problem is your cast, which is what is known as an unsafe typecast. An unsafe typecast from an interface reference to an object has ill-defined behaviour in older versions of Delphi. However, the behaviour is well-defined in modern Delphi. The documentation says more.

So, the basic problem is that your expectations for the behaviour are not compatible with the Delphi 7 version of the language.

If you get the interface to return the ID you will find that the interface you are creating is as expected.

program InterfaceDemo;

{$APPTYPE CONSOLE}

uses
  Classes;

type
  IMyIntf = interface
    function GetID: Integer;
  end;

  TImplementingObject = class(TInterfacedObject, IMyIntf)
  private
    FID: Integer;
    function GetID: Integer;
  public
    constructor Create;
  end;

{ TImplementingObject }

constructor TImplementingObject.Create;
begin
  FID := Random(100);
  Writeln(FID);
end;

function TImplementingObject.GetID: Integer;
begin
  Result := FID;
end;

var
  MyIntf: IMyIntf;

begin
  Randomize;
  MyIntf := TImplementingObject.Create;
  Writeln(MyIntf.GetID);
  Readln;
end.

It's rather unusual to ask for the implementing object from an interface. To do so suggests that there is a problem with your design. Should you really need to do so there are a few options:

• In modern Delphi you can use the type-safe case with the as operator.
• In older Delphi versions there are hacks that retrieve the implementing object: Casting a delphi interface to its implementation class without modifying the interface
• You could add a function to the interface that returns the implementing object.

The latter option works in all versions of Delphi and does so without resorting to subterfuge.