刚学Delphi碰到有关问题

刚学Delphi碰到问题
我看的是Delphi   7从入门到精通其中有个例子这样的
unit   Unit1;

interface

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

type
    TForm1   =   class(TForm)
        procedure   FormMouseDown(Sender:   TObject;   Button:   TMouseButton;
            Shift:   TShiftState;   X,   Y:   Integer);
    private
        {   Private   declarations   }
    public
        {   Public   declarations   }
    end;

var
    Form1:   TForm1;

implementation

{$R   *.dfm}

procedure   TForm1.FormMouseDown(Sender:   TObject;   Button:   TMouseButton;
    Shift:   TShiftState;   X,   Y:   Integer);
var
    Btn:TButton;           运行后显示这里出错
begin
    Btn:=TButton.Create(Self);
    Btn.Parent:=Self;
    Btn.Left:=X;
    Btn.Top:=Y;
    Btn.Width:=Btn.Width=50;
    Btn.Caption:=Format( 'Button   at   %d,%d ',[X,Y]);
end;

end.

为什么运行是出错的？我是按书上写的

[解决办法]
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;
~~~~~~~~添加

Btn.Width:=Btn.Width=50;
~~~~~~~~~~删去