一个使用多线程写同一个文件的程序示例，请大家帮忙调试

//程序很简单，内容如下：


unit Main_Unit;

interface

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

type TWriteThread = class(TThread)

private
//
protected
procedure Execute;
public
//
end;

type
TMainForm = class(TForm)
Button1: TButton;
Button2: TButton;
CheckBox1: TCheckBox;
Memo1: TMemo;
procedure Button2Click(Sender: TObject);

private
{ Private declarations }
public
{ Public declarations }
procedure ThreadDone(Sender : TObject);
end;

var
MainForm: TMainForm;
DoneFlags : integer;
tempfile : file;
cs:TRTLCriticalSection;
filename : string;
pos : longint;

implementation

{$R *.dfm}

{ TWriteThread }

{constructor TWriteThread.create;
begin
//
end; }

procedure TWriteThread.Execute; //每个线程都向today.txt写入内容:"f"
var
i ,temp_len : integer;
buf : array[1..100] of char;
begin
OnTerminate:=MainForm.ThreadDone;
EnterCriticalSection(cs);
filename := 'D:\Documents and Settings\Administrator\桌面\today.txt';
for i:=1 to 100 do
begin
assignfile(tempfile,filename);
if FileExists(filename)=true then
begin
reset(tempfile,1);
pos:=filesize(tempfile);
end
else
begin
rewrite(tempfile,1);
pos:=0;
end;
seek(tempfile,pos);

FillChar(buf,sizeof(buf),'f');
temp_len := sizeof(buf);
blockwrite(tempfile,buf,temp_len);
end;
LeaveCriticalSection(cs);
end;

{ TMainForm }

procedure TMainForm.ThreadDone;
var
i : integer;
begin
//inherited;
inc(DoneFlags);
if DoneFlags = 10 then
begin //make sure 10 threads all finished
closefile(tempfile);
DeleteCriticalSection(cs);
end
end;

procedure TMainForm.Button2Click(Sender: TObject);
var
TempWriteThds: array[1..10] of TWriteThread;
i : integer;
Handel_Thd : array[1..10] of THandle;
begin
InitializeCriticalSection(cs);
for i:=1 to 10 do //创建十个线程
begin
TempWriteThds[i] :=TWriteThread.Create(false);
sleep(10);
Handel_Thd[i] := TempWriteThds[i].Handle;
end;
end;

end.



我在运行时，出现：Abstract error！请大家帮忙看一下是什么问题。谢谢！