我在同一个TOwnedCollection中添加自己的TCollectionItem类（继承自TCollectionItem）时遇到问题。
我参考了Indy的IdMessageParts.pas作为TIdMessagePart的推荐。所以我一定是错过了什么，因为我得到了一个“无效的类类型”错误时，添加TMyItem2。
我希望MyItems: TOwnedCollection能够存储TMyItem、TMyItem2、TMyItem3。但是在添加TMyItem2和TMyItem3时，我收到了“Invalid typecast”错误（仅可接受TMyItem）。
我错过什么了吗？

TMyItem = class(TCollectionItem)
private
  FLabelName: string;
public
  constructor Create(Collection: TCollection); override;
  destructor Destroy; override;
published
  property LabelName: string read FLabelName write FLabelName;
end;

TMyItem2 = class(TMyItem)
private
  FCaption: string;
published
  property Caption: string read FCaption write FCaption;
end;

TMyItem3 = class(TMyItem)
private
  FCaption3: string;
published
  property Caption3: string read FCaption3 write FCaption3;
end;

TMyItems = class(TOwnedCollection)
private
  function GetMyItem(aIndex: Integer): TMyItem;
protected
  constructor Create(aOwner: TAsapListview);
  function GetOwner: TPersistent; override;
public
  function Add: TMyItem;
  function IndexOf(aFieldName: string): Integer;
  function MyItemByFieldName(aFieldName: string): TMyItem;
  property Items[aIndex: Integer]: TMyItem read GetMyItem; default;
end;

// NOTE: in idMessageParts.pas the add is defined this way
// which I don't quite understand
{
function TIdMessageParts.Add: TIdMessagePart;
begin
  // This helps prevent TIdMessagePart from being added
  Result := nil;
end;
}

// this is the calling code
with MyItems.Add do
begin
  LabelName := 'test';  // works
end;
// Error of invalid typecast occurred here
with MyItems.Add as TMyItem2 do  // typecast error here
begin
  LabelName := 'label item2';
  Caption := 'caption item2';
end;
with MyItems.Add as TMyItem3 do  // typecast error here
begin
  LabelName := 'label item2';
  Caption := 'caption item2';
  Caption3 := 'caption3 item2';
end;