I can't compile class with an interface - delphi

Question

I am trying to create a class that implements an interface but I get these errors:
[dcc32 Error] dl_tPA_MailJournal.pas(10): E2291 Missing implementation of interface method IInterface.QueryInterface
[dcc32 Error] dl_tPA_MailJournal.pas(10): E2291 Missing implementation of interface method IInterface._AddRef
[dcc32 Error] dl_tPA_MailJournal.pas(10): E2291 Missing implementation of interface method IInterface._Release
[dcc32 Fatal Error] MainUnit.pas(8): F2063 Could not compile used unit 'dl_tPA_MailJournal.pas'
The code is:
unit dl_tPA_MailJournal;
interface
uses
Windows,
Generics.Collections,
SysUtils,
uInterfaces;
type
TtPA_MailJournal = class(TObject, ITable)
public
function GetanQId: integer;
procedure SetanQId(const Value: integer);
function GetadDate: TDateTime;
procedure SetadDate(const Value: TDateTime);
function toList: TList<string>;
constructor Create(aId : Integer; aDate : TDateTime);
private
property anQId : integer read GetanQId write SetanQId;
property adDate : TDateTime read GetadDate write SetadDate;
end;
implementation
{ TtPA_MailJournal }
constructor TtPA_MailJournal.Create(aId : Integer; aDate : TDateTime);
begin
SetanQId(aId);
SetadDate(aDate);
end;
function TtPA_MailJournal.GetadDate: TDateTime;
begin
Result := adDate;
end;
function TtPA_MailJournal.GetanQId: integer;
begin
Result := anQId ;
end;
procedure TtPA_MailJournal.SetadDate(const Value: TDateTime);
begin
adDate := Value;
end;
procedure TtPA_MailJournal.SetanQId(const Value: integer);
begin
anQId := Value;
end;
function TtPA_MailJournal.toList: TList<string>;
var
aListTable: TList<TtPA_MailJournal>;
aTable: TtPA_MailJournal;
aListString: TList<String>;
begin
aTable.Create(1,now);
aListTable.Add(aTable);
aTable.Create(2,now);
aListTable.Add(aTable);
aListString.Add(aListTable.ToString);
Result := aListString;
end;
end.
And the interface is:
unit uInterfaces;
interface
uses
Generics.Collections;
type
ITable = Interface
['{6CED8DCE-9CC7-491F-8D93-996BE8E4D388}']
function toList: TList<string>;
end;
implementation
end.

Answer 1

The problem is that you use TObject as the parent for your class. You should use TInterfacedObject instead.
In Delphi, every interface inherits from IInterface at therefore has, at least, the following 3 methods:
_AddRef
_Release
QueryInterface
You must implement these 3 methods, either by implementing them yourself or by inheriting from a base object that includes these methods.
Because you inherit from TObject, but you are not implementing these 3 methods, you get a compilation error. If you read the compiler error, you will see that it actually spells out this omission for you.
TInterfacedObject has already implemented these methods for you.
Other base objects that implement IInterface (aka IUnknown) are: TAggregatedObject and TContainedObject. However these are special purpose vehicles, only to be used if you really know what you're doing.
Change the definition of your class to
TTPA_MailJournal = class(TInterfacedObject, ITable)
And your code will compile.
See Delphi basics for more info.