delphi面向对象编程问题

我想学习面向对象编程，想做一个系统自带的“挖地雷”那个小游戏，我想把地雷定义一个类，我在单独的单元DiLei中这样定义：
unit Dilei;

interface
uses
Buttons;
type
TDiLei = class //地雷类别
private
{ Private declarations }
public
btn: TSpeedButton; {按钮}
btnLeft: Integer;{left}
btnTop: Integer;
btnWidth: Integer;
btnHeight: Integer;
constructor create(l,t: Integer);overload;
{ Public declarations }
end;

implementation



constructor TDiLei.create(l,t: Integer);
begin
Self.btn := TSpeedButton.Create(Self);
Self.btn.Width := 22;
Self.btn.Height := 22;
Self.btn.Left := l;
Self.btn.Top := t;
Self.btn.Show
end;

end.

//////////////////////////////////////////////////////////////////////////////////////
然后在窗口OnShow的时候全部创建这些地雷：
unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, Buttons,Dilei;

type
TForm1 = class(TForm)
procedure FormShow(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
Form1: TForm1;
dl: array [1..20,1..20] of TDiLei;

implementation

{$R *.dfm}

procedure TForm1.FormShow(Sender: TObject);
var
i,j :Integer;
begin
for i:= 1 to 20 do
begin
for j := 1 to 20 do
begin
dl[i,j] := TDiLei.create((j-1)*22,(i-1)*22);
end;
end;
end;

end.
但是编译说我的那个"Self.btn := TSpeedButton.Create(Self);"中的self不兼容，怎么做才能显示出来？