为什么继承自TGRAPHICCONTROL的控件不能画自己呢？为什么说‘’IS NOT A CONTROL？(17分)

type
aaa = class(TgraphicControl)
private
{ Private declarations }
protected
{ Protected declarations }
public
constructor create(aowner:tcomponent);override;
{ Public declarations }
published
{ Published declarations }
end;

procedure Register;

implementation

constructor aaa.create(aowner:tcomponent);
begin
inherited create(aowner);
canvas.MoveTo(left,Top);
canvas.LineTo(Left+Width,Top);
canvas.LineTo(Left+Width,Top+Height);
canvas.LineTo(Left,Top+Height);
canvas.LineTo(Left,Top);
end;

修改如下：
type
aaa = class(TgraphicControl)
private
{ Private declarations }
protected
procedure Paint;override;//注意，添加函数
public
constructor create(aowner:tcomponent);override;
{ Public declarations }
published
{ Published declarations }
end;

procedure Register;

implementation

constructor aaa.create(aowner:tcomponent);
begin
inherited create(aowner);
//其他初始化工作，在这里是空的，可以不Override
end;
procedure aaa.Paint;//在这里才能画
begin
canvas.MoveTo(left,Top);
canvas.LineTo(Left+Width,Top);
canvas.LineTo(Left+Width,Top+Height);
canvas.LineTo(Left,Top+Height);
canvas.LineTo(Left,Top);
end;

接受答案了.