如何继承twincontrol，画一个矩形控件？

Question

我最终想开发一个图形程序，现在想把各种基本图形做成控件
为了能在控件的运行期模拟设计期的效果，学习网上的例子，从Twincontrol继承
我下面写的出错：“canvas does not allow drawing”
为什么？怎么办呢？
unit myfirst1;interfaceuses
  SysUtils, Classes, Controls,Graphics;type
  tmyfirst1 = class(TWinControl)
  private
    Fcanvas:Tcanvas;
    { Private declarations }
  protected
    { Protected declarations }
  public
    constructor create(Aowner:Tcomponent);override;
    destructor destroy;override;
    { Public declarations }
  published
    { Published declarations }
  end;procedure Register;implementation
constructor Tmyfirst1.create(Aowner:Tcomponent);
begin
  inherited create(Aowner);
  Fcanvas:=Tcanvas.Create;
  fcanvas.Lock;
  fcanvas.Rectangle(30,30,80,60);
  fcanvas.Unlock;end;
destructor Tmyfirst1.destroy;
begin
  inherited destroy;
  fcanvas.Free;
end;
procedure Register;
begin
  RegisterComponents('user', [tmyfirst1]);
end;end.

Answer 1

pen,color有用么？我试了加上
fcanvas.pen.color没用