Question

Let’s say we have interface I, object O and a container sl:

type
  rootInterface = Interface
  end;

  childOfRootInterface = interface(rootInterface)
  end;

  ObjectImplementsRootInterface = class(TObject, I)
  end;

  ObjectImplementsChildOfRootInterface = class(TOtherObject, I2);
  end;

var
  aStringList: TStringList;

Assuming that sl is initialized.

If we do

aStringList.Items.AddObject('root', ObjectImplementsRootInterface );

then we can’t do

ObjectImplementsRootInterface  := I(aStringList.Items.Objects([0]));

because Delphi forbids this. so trying:

ObjectImplementsRootInterface  := TObject(aStringList.Items.Objects([0]));

However, we inserted an childOfRootInterface and we want back an childOfRootInterface so we do

ObjectImplementsChildOfRootInterface  := childOfRootInterface (sl.Items.Objects([0]));

But ObjectImplementsChildOfRootInterface and ObjectImplementsRootInterface have different supers!

Although they support the same interface, we can’t do anything.

In java you can pass around objects, as long as they support interface, In java it would be possible, so now after all this planning; what can I do in Delphi?

Update: changed the names, however the example might got over me and won, can’t find right and left.

Answer 1

Here is a working code example:

program Project9054004;

{$APPTYPE CONSOLE}

uses
  Classes, SysUtils, Dialogs;

type  
  I = interface
  ['{6D39CF0A-839A-4DA3-B058-52E424702652}']
  end;

  O = class(TInterfacedObject, I)
  end;

var
  SL: TStrings;
  TempO: O;
  TempI: I;
begin
  ReportMemoryLeaksOnShutdown := True;

  SL := TStringlist.Create;
  try
    TempO := O.Create;
    SL.AddObject('a', TempO);

    if Supports(SL.Objects[0], I, TempI) then
    begin
      // use I
      TempI._AddRef;
      TempI._Release;
    end;

    ShowMessage(SL.Objects[0].ClassName);

  finally
    SL.Free;
  end;
end.

To avoid a hard type cast I use a GUID for the interface.

Supports can be used for a safe test and assignment of the object to a variable of type I. Note that type names are a bit confusing in your example.

Hard typecasting to an interface is not possible for items in the object list.