如何在TOwnedCollection中添加TCollectionItem的2倍派生类？

type
  TMyCollectionItem = class(TCollectionItem)
  private
    FExtraProperty: string;
  public
    property ExtraProperty: string read FExtraProperty write FExtraProperty;
    procedure ExtraMethod;
  end;

  TOwnedCollection = class(TOwnedCollection)
  public
    function AddMyCollectionItem: TMyCollectionItem;
  end;

function TOwnedCollection.AddMyCollectionItem: TMyCollectionItem;
begin
  Result := TMyCollectionItem(inherited Add);
end;

var
  MyOwnedCollection: TOwnedCollection;
  MyItem: TMyCollectionItem;
begin
  MyOwnedCollection := TOwnedCollection.Create(nil);
  try
    MyItem := MyOwnedCollection.AddMyCollectionItem;
    MyItem.ExtraProperty := 'Some value';
    MyItem.ExtraMethod;
  finally
    MyOwnedCollection.Free;
  end;
end;