Question

In one file, I have a baseclass with a ID-property:

type   TBase = class(TObject)     private       FID: integer;     public       property ID: integer read FID write SetID;     end;   

In a second file, I have another class, descending from TBase. By accident, or ignorance or what ever, a new property/field with the same name as an exsisting property/field was made.

type   TSub = class(TBase)   private     FID: Longword;   public          property ID: Longword read FID write FID;   end; 

The second ID-fields is of course renamed, but why does the compiler allow this?
When accessing ID in code – which ID-field is used?

Answer 1

Delphi allows this to ‘hide’ your old properties and introduce properties with the same name that operate differently. I’ve used this for example to create my own typesafe TList descendants for a specific class or record:

type   TMyList = class(TList)   private     function GetItem(Index: Integer): TMyObject;     procedure SetItem(Index: Integer; Value: TMyObject);   public     property Items[Index: Integer]: TMyObject read GetItem write SetItem;   end;    function TMyList.GetItem(Index: Integer): TMyObject;   begin     Result := TMyObject(inherited Items[Index]);   end;    procedure SetItem(Index: Integer; Value: TMyObject);   begin     inherited Items[Index] := Value;   end; 

Ofcourse, with Delphi 2009 and generics, it’s a lot easier now.

Which ID you are accessing depends on the place you are calling ID from.

procedure TSub.Foo; begin   ID := 5; //TSub.ID   inherited ID := 6 //TBase.ID end;  procedure TBase.FooBar; begin   ID := 5; //TBase.ID end;  var   Sub: TSub;   Base: TBase; begin   Sub := TSub.Create;   try     Sub.ID := 1; //assign 1 to TSub.ID     TBase(Sub).ID := 2; //assign 2 to TBase.ID     WriteLn(Sub.ID); //writes 1     WriteLn(TBase(Sub).ID); //writes 2     Base := Sub;     WriteLn(Base.ID); //writes 2   finally     Sub.Free;   end; end;