如何为通用集合项定义模板？

Question

我正在尝试为通用列表定义我自己的项目类型。所有项目都需要有一个owner 属性。

type 
  TGenericCollection<TGenericCollectionItem: class> = class(TObjectList<TGenericCollectionItem>)
  protected
    procedure Notify(const Value: TGenericCollectionItem; Action: TCollectionNotification); override;
  public
    procedure Assign(Source: TGenericCollection<TGenericCollectionItem>); virtual;
  end;

  TGenericCollectionItem = class
  private
    FOwner: TGenericCollection<TGenericCollectionItem>;  //another class can change this value if it was declared in the same unit
  protected
    function GetOwner: TGenericCollection<TGenericCollectionItem>; virtual;
  public
    constructor Create; virtual;
    property Owner: TGenericCollection<TGenericCollectionItem> read GetOwner;
  end;

implementation

{ TGenericCollection<TGenericCollectionItem> }
procedure TGenericCollection<TGenericCollectionItem>.Assign(
  Source: TGenericCollection<TGenericCollectionItem>);
begin
  AddRange(Source.ToArray);
end;

procedure TGenericCollection<TGenericCollectionItem>.Notify(
  const Value: TGenericCollectionItem; Action: TCollectionNotification);
begin
  inherited;

  if (Action = cnAdded) then
  begin
    OutputDebugString(PChar(Value.ClassName)); // Debug Output: THandingItem Process GenericCollection.exe (2424)
    OutputDebugString(PChar(Self.ClassName));  // Debug Output: THandingList Process GenericCollection.exe (2424)
//    Value.FOwner := Self;  // Error: [dcc32 Error] Unit2.pas(46): E2003 Undeclared identifier: 'FOwner'
//    (Value as TGenericCollectionItem).FOwner := Self;   // Error: [dcc32 Error] Unit2.pas(46): E2003 Undeclared identifier: 'FOwner'
//    (Value as TGenericCollectionItem<T>).FOwner := Self;   // Undeclared identifier 'T'
  end;
end;

当我尝试更改 FOwner 值时，编译器会说 FOwner 是未声明的标识符。 THandingList 继承自 TGenericCollection。 THandingItem 继承自 TGenericCollectionItem。

我做错了什么？ Sample project

---

UPD：TGenericCollectionItem 已在代码示例中更改。

---

控制台应用：

program Project1;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.SysUtils,
  System.Generics.Collections;

type
  TGenericCollection<TGenericCollectionItem: class> = class(TObjectList<TGenericCollectionItem>)
  protected
    procedure Notify(const Value: TGenericCollectionItem; Action: TCollectionNotification); override;
  end;

  TGenericCollectionItem = class
  public
    Owner: TGenericCollection<TGenericCollectionItem>;
  end;

  procedure TGenericCollection<TGenericCollectionItem>.Notify(
    const Value: TGenericCollectionItem; Action: TCollectionNotification);
  begin
    inherited;

    if (Action = cnAdded) then
      Value.Owner := Self;
  end;

var
  GenericCollectionItem: TGenericCollectionItem;
  GenericCollection: TGenericCollection<TGenericCollectionItem>;
begin
  try
    { TODO -oUser -cConsole Main : Insert code here }
    GenericCollection := TGenericCollection<TGenericCollectionItem>.Create;
    try
      GenericCollectionItem := TGenericCollectionItem.Create;
      GenericCollection.Add(GenericCollectionItem);
    finally
      GenericCollection.Free;
    end;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

Answer 1

哦，这令人困惑。您已将TGenericCollectionItem 用作通用参数名称和类型名称。在TGenericCollection 代码中，名称TGenericCollectionItem 指的是泛型类型参数，而不是同名的类。您对TGenericCollectionItem 的所有了解就是它是一个类。也就是说，它源自TObject。因此没有名为Owner 或确实FOwner 的成员。

改变

TGenericCollection<TGenericCollectionItem: class>

到

TGenericCollection<T: TGenericCollectionItem>

请注意，您可能需要转发声明 TGenericCollectionItem。

更新：啊，由于 Delphi 泛型的设计缺陷，当相互引用的类之一是泛型时，前向声明将不起作用。见How to set a forward declaration with generic types under Delphi 2010?