delphi - "Control ' ' has no parent window"错误

Question

当我将控件放在表单上时，出现此错误。错误出现在这里:

  TAssociateFileExt = class(TGroupBox)
  private
  protected
  public
    btnAssociate   : TButton;
    constructor Create(aOwner: TComponent); override;
  end;

constructor TAssociateFileExt.Create(aOwner: TComponent);
begin
 inherited Create(aOwner);
 Caption:= '';                       <-------- error here
 ClientHeight:= 125;                 <-------- and here
 ClientWidth := 170;
 DoubleBuffered:= TRUE;

 btnAssociate:= TButton.Create(Self);
 btnAssociate.Parent:= Self;
 btnAssociate.Visible:= TRUE;
 btnAssociate.Left:= 17;
 btnAssociate.Top:= 26;
 btnAssociate.Width:= 142;
 btnAssociate.Height:= 25;
 btnAssociate.Hint:= 'Associate this application with its files. When you double click a file this program will automatically start and load that file.';
 btnAssociate.Caption:= 'Associate';
 btnAssociate.DoubleBuffered:= TRUE;
 btnAssociate.ParentDoubleBuffered:= FALSE;
 btnAssociate.TabOrder:= 0;
 btnAssociate.OnClick:= btnAssociateClick;

 btnAssociateDel:= TButton.Create(Self);
 btnAssociateDel.Parent:= Self;
 btnAssociateDel.Visible:= TRUE;
 btnAssociateDel.Left:= 17;
 btnAssociateDel.Top:= 53;
 btnAssociateDel.Width:= 142;
 btnAssociateDel.Height:= 25;
 btnAssociateDel.Hint:= 'Remove association';
 btnAssociateDel.Caption:= 'Remove association';
 btnAssociateDel.DoubleBuffered:= TRUE;
 btnAssociateDel.ParentDoubleBuffered:= FALSE;
 btnAssociateDel.TabOrder:= 1;
 btnAssociateDel.OnClick:= btnAssociateDelClick;

 chkAllUsers:= TCheckBox.Create(Self);
 chkAllUsers.Parent:= Self;
 chkAllUsers.Visible:= TRUE;
 chkAllUsers.Left:= 31;
 chkAllUsers.Top:= 97;
 chkAllUsers.Width:= 115;
 chkAllUsers.Height:= 17;
 chkAllUsers.Hint:= 'Please note that if you want to do this for all users then you need administrator/elevated rights.';
 chkAllUsers.Caption:= 'Do this for all users';
 chkAllUsers.DoubleBuffered:= TRUE;
 chkAllUsers.ParentDoubleBuffered:= FALSE;
 chkAllUsers.TabOrder:= 2;
 chkAllUsers.OnClick:= chkAllUsersClick;
end;

答案可能是“标题需要有效的窗口句柄”。正确的？然而，David Intersimone ( here ) 的一篇文章说，在构造函数中设置 Caption 是可以的。

1. 这篇文章有错吗？
2. 我应该将代码(Caption 和 TButton.Create)移到 CreateWnd 中(因为 AfterConstruction 不是一个好地方)吗？问题是 CreateWnd 可以被多次调用:“当首次创建控件或必须销毁并重新创建底层屏幕对象以反射(reflect)属性更改时，会自动调用 CreateWnd。”

<小时/>

更新:
按照 J... 的建议，添加 (aOwner: TComponent) 到构造函数的声明(在实现中)后，错误移至下一行 (clientheight:= 90);

Answer 1

我将代码移至 CreateWindowHandle 中。现在可以了。完整代码:

UNIT cAssociateExt;

INTERFACE

USES
  Windows, Messages, SysUtils, Classes, Controls, ExtCtrls, Forms, StdCtrls;

TYPE
  TAssociateFileExt = class(TGroupBox)
  private
  protected
  public
    btnAssociate   : TButton;
    btnAssociateDel: TButton;
    chkAllUsers    : TCheckBox;
    constructor Create(aOwner: TComponent); override;
    procedure AfterConstruction;  override;
    procedure CreateWindowHandle(const Params: TCreateParams); override;
    ...
  published
  end;


procedure Register;


IMPLEMENTATION


procedure TAssociateFileExt.AfterConstruction;
begin
 inherited;         //Not a good place here
end;


procedure TAssociateFileExt.CreateWindowHandle(const Params: TCreateParams);
begin
 inherited;

 //DO NOT CREATE CONTROLS HERE! See: Sertac Akyuz's comment

 Caption:= '';    
 ClientHeight:= 125;
 ClientWidth := 170;
end;



constructor TAssociateFileExt.Create(aOwner: TComponent);
begin
 inherited Create(aOwner);
 DoubleBuffered:= TRUE;

 btnAssociate:= TButton.Create(Self);
 btnAssociate.Parent:= Self;
 btnAssociate.Visible:= TRUE;
 btnAssociate.Left:= 17;
 ...
end;