Delphi 10.2 Tokyo - Casting object provided by interface - delphi

Question

I'm trying to convert my aplliction from Delphi XE8 to 10.2 Tokyo. I'm getting strange runtime exeptions with casting objects provided by interfafce acrocss packages ( bpl's). when I try to cast objects with "as" keyword I'm getting
this exception during runtime:
Project Project1.exe raised exception class EInvalidCast with message
'Invalid class typecast'
Here is the code :
Interface in a separte package Plugin_interface.bpl :
unit MainIntf;
interface
Type IMainInft = interface
['{FE08C4A2-069C-4B8C-BB1B-445348CAB6A0}']
function GetForm : TObject;
end;
implementation
end.
Interface implamentation provided in Project1.exe :
unit MainImpl;
interface
uses MainIntf;
Type TMain = class(TInterfacedObject,IInterface,IMainInft)
function GetForm : TObject;
end;
implementation
uses unit1;
function TMain.GetForm: TObject ;
begin
result:=Form1; // interafce is implemented on the main form so Form1 is rechable form here
end;
end.
And finally in another package "plugin.bpl" I'm trying to obtain object from interface :
unit Plugin_main;
interface
uses Mainintf, Vcl.Forms;
type TPlugin = class (Tobject)
IIMyRefernceToMianIntf: IMainInft;
end;
function RegisterPlugin(AMainIntf: IMainInft): TForm ; export;
procedure UnRegisterPlugin; export;
exports
RegisterPlugin,
UnRegisterPlugin;
var
Plugin_obj: TPlugin;
implementation
uses vcl.Dialogs,System.Classes ;
function RegisterPlugin(AMainIntf: IMainInft): TForm ;
var
MyForm : TForm ;
begin
Plugin_obj:=TPlugin.Create;
Plugin_obj.IIMyRefernceToMianIntf:=AMainIntf;
if AMainIntf.GetForm is TForm then
Showmessage ('Great it is a Tform') // will not happen
else
Showmessage ('Sorry it is not Tform'); // will happen
if TComponent (AMainIntf.GetForm).Classname='TForm1' then
Showmessage ('What ?? It is TForm1 decsendant from TForm so is it TForm after all ?!'); // will happen
// result:= AMainIntf.GetForm as TForm -- This will rise na exception
result:= TForm( AMainIntf.GetForm) ; // this will work
end;
procedure UnRegisterPlugin;
begin
Plugin_obj.Free;
end;
end.
Why cant I use "as" and "is" keyword .
Only hard catsing will do, but i hate to do it .
on XE8 compiler everything worked as expected - problem exists on XE 10.2 tokyo compiler

Answer 1

The "is" keyword checks the actual objects to see it is of the type you are asking. So, checking for this:
if AMainIntf.GetForm is TForm then
Showmessage ('Great it is a Tform') // will not happen
does not happen because GetForm returns TObject and not TForm. Checking with "is" means, also, that you are checking for castability, i.e. the ability to use the "as" keyword. Since, the "is" check fails, that command fails as well:
result:= AMainIntf.GetForm as TForm;
Your next option here is to hard-cast GetForm the way you do it:
TForm(AMainIntf.GetForm);
which works because this casting does not check whether GetForm is of TForm type. Since you return a form in TMain, this hard-casting is safe bet for you.
Having said that, however, why don't you return TForm directly rather than TObject? Do you use IMainInft in other classes that return other types than TForm?