TTaskFormList = class(TObjectList)
  private
    { Private declarations }
    function GetItem(Index: Integer): TTaskForm;
    procedure SetItem(Index: Integer; Value: TTaskForm);
  protected
    { Protected declarations }
  public
    { Public declarations }
    property Items[Index: Integer]: TTaskForm read GetItem write SetItem; default;
  end;

function TTaskFormList.GetItem(Index: Integer): TTaskForm;
begin
  Result := TTaskForm(inherited GetItem(Index));
end;

procedure TTaskFormList.SetItem(Index: Integer; Value: TTaskForm);
begin
  inherited SetItem(Index, Value);
end;

  TTaskForm = class(TObject)
  ...
  TAaaTaskForm = class(TTaskForm)
  ...
  TBbbTaskForm = class(TTaskForm)
  ...

  TTaskFormList = class(TObjectList)
  private
    { Private declarations }
    FTaskFormClass: TTaskFormClass;
    function CheckObject(AObject: TObject): Boolean;
    function GetItem(Index: Integer): TTaskForm;
    procedure SetItem(Index: Integer; Value: TTaskForm);
  protected
    { Protected declarations }
  public
    { Public declarations }
    constructor Create(ATaskFormClass: TTaskFormClass); overload;
    constructor Create(AOwnsObjects: Boolean; ATaskFormClass: TTaskFormClass); overload;
    function Add(AObject: TTaskForm): Integer;
    procedure Insert(Index: Integer; AObject: TTaskForm);
    property Items[Index: Integer]: TTaskForm read GetItem write SetItem; default;
  end;

function TTaskFormList.CheckObject(AObject: TObject): Boolean;
begin
  Result := Assigned(AObject) and (AObject is FTaskFormClass);
end;

function TTaskFormList.GetItem(Index: Integer): TTaskForm;
begin
  Result := TTaskForm(inherited GetItem(Index));
end;

procedure TTaskFormList.SetItem(Index: Integer; Value: TTaskForm);
begin
  if not CheckObject(Value) then Exit;
  if IndexOf(Value) <> - 1  then Exit;
  inherited SetItem(Index, Value);
end;

constructor TTaskFormList.Create(ATaskFormClass: TTaskFormClass);
begin
  inherited Create;
  FTaskFormClass := ATaskFormClass;
end;

constructor TTaskFormList.Create(AOwnsObjects: Boolean; ATaskFormClass: TTaskFormClass);
begin
  inherited Create(AOwnsObjects);
  FTaskFormClass := ATaskFormClass;
end;

function TTaskFormList.Add(AObject: TTaskForm): Integer;
begin
  if not CheckObject(AObject) then begin Result := - 1 ; Exit; end;
  Result := IndexOf(AObject); if Result <> - 1  then Exit;
  Result := inherited Add(AObject);
end;

procedure TTaskFormList.Insert(Index: Integer; AObject: TTaskForm);
begin
  if not CheckObject(AObject) then Exit;
  if IndexOf(AObject) <> - 1  then Exit;
  inherited Insert(Index, AObject);
end;

function TTaskFormList.CheckObject(AObject: TObject): Boolean;
begin
  Result := Assigned(AObject) and (AObject is FTaskFormClass); // 1 ) не заработает;  2 ) если нужна ошибка
end;

function TTaskFormList.CheckObject(AObject: TObject): Boolean;
begin
  Result := Assigned(AObject) and (AObject.ClassName = FTaskFormClass.ClassName);
  if not Result then Exception.Create('Boroda');
end;