如何在 Delphi 中获取具有通用类型的对象列表

Question

我有几个类如下：

Type
  TSystemBaseEntity = class(TPersistent)
  private 
    FID: integer;
  public
    property ID: integer read FID write FID;
  end;

  TProcessHeaderEntity = class(TSystemBaseEntity)
  private
    FHeaderDate: TDateTime;
  public
     property HeaderDate: TDateTime read FHeaderDate write FHeaderDate;
  end;

  TInvoiceHeaderEntity = class(TProcessHeaderEntity)
  private 
    FCustomerId: integer;
  public
    property CustomerId: integer read FCustomerId write FCustomerId;
  end;


  TRequestHeaderEntity = class(TProcessHeaderEntity)
  private 
    FWarehouseId: integer;
  public
    property WarehouseId: integer read FWarehouseId write FWarehouseId;
  end;


  TDataList = class(TPersistent)
  private
    FValues: TObjectList<TSystemBaseEntity>;
  protected
    function SetCaption: string; virtual;
  public
    procedure Execute; virtual; abstract;
    property Values: TObjectList<TSystemBaseEntity> read FValues;
  end;

  TInvoice = class(TDataList)
  public
    procedure Execute; override;
  end;

如何通过 Values 属性获取具有从 TSystemBaseEntity 继承的泛型类型的对象列表？

例如，发票列表 (TInvoiceHeaderEntity) 或请求列表 (TRequestHeaderEntity) 以及对其属性的访问权限。

Answer 1

如果您想要一个包含 TSystemBaseEntity 的一个特定子类的 TDataList，请将 TDataList 定义为泛型类，并带有 type constraint

这被定义为

  TDataList<T:TSystemBaseEntity> = class(TPersistent)
  private
    FValues: TObjectList<T>;
  protected
    function SetCaption: string; virtual;
  public
    procedure Execute; virtual; abstract;
    property Values: TObjectList<T> read FValues;
  end;

并用作

  TInvoice = class(TDataList<TInvoiceHeaderEntity>)
  public
    procedure Execute; override;
  end;

procedure TInvoice.Execute
var
  InvoiceHeader: TInvoiceHeaderEntity;
begin
  for InvoiceHeader in Values do
    ...
end;

如果您想要一种类型的 TDataList，其成员为 TObjectList<TSystemBaseEntity>，则不能将其用作 TObjectList<TInvoiceHeaderEntity> 等。请查看 Wikipedia 的介绍，但还要考虑允许以下内容在这样的计划下：

procedure DoBadThingsWithGenerics(aDataList: TDataList);
var
  myInvoices: TObjectList<TInvoiceHeaderEntity>
  myRequests: TObjectList<TRequestHeaderEntity>
  Request: TRequestHeaderEntity;
begin
  myInvoices := aDataList.Values<TInvoiceHeaderEntity>;
  myRequests := aDataList.Values<TRequestHeaderEntity>;

  // Some code ...

  myInvoices.Add( TInvoiceHeaderEntity.Create );

  // Some more code...

  for Request in myRequests do
    // Oops, we have a TInvoiceHeaderEntity pretending to be a TRequestHeaderEntity
end;

你可以在这里做的是有一个过程，它接受一个新的后代类型列表，过滤值列表中适当类型的元素并将它们添加到新列表中。

procedure TDataList.FilterByType<S:TSystemBaseEntity> ( intoList: TObjectList<S> );
var
  Value: TSystemBaseEntity;
begin
  for Value in Values do
    if Value is S then
      intoList.Add( Value as S );
end;