首先，来看一下定制文件流（Tmystream）的声明：
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type
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Tmystream=class(Tfilestream)
private
fkey:string;
public
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constructor create(const filename:string;mode:word);
function read(var buffer;count:longint):longint;override;
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function write(const buffer;count:longint):longint;override; 字串2
property key:string read fkey write fkey ; 字串3
end;
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---- 在Tmystream的声名中，我们对read、write两个方法进行了重载，并添加了一个新的特性key，用以存储对文件进行加密时所需的密码。为实现文件读写的加密，在write方法中，将key的每个字符依次与buffer中的字符相加，将得到的结果写入文件，实现加密；在read方法中，将读出的内容依次与key的每个字符相减，实现解密。加密及解密的方法多种多样，读者可以通过改写相关代码，得到不同的加密方法。 字串8
程序清单如下：
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function Tmystream.write(const buffer;count:longint):longint;
var
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Pbu,Pmy,mykey:pchar; 字串4
i,enc:integer;
begin
getmem(pmy,count); //为pmy分配内存
mykey:=pchar(key); //将key转换为pchar指针
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try
pbu:=pchar(@buffer); //将buffer转换为pchar指针 字串5
for i:=0 to count-1 do 字串5
//将key的每个字符以此与buffer的每个字符循环相加,结果放入pmy指向的内存区
begin
enc:=(ord(pbu[i])+ord(mykey[(i mod length(key))])) mod 256;
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Pmy[i]:=char(enc); 字串4
end;
result:=inherited write(Pmy^,count); //将pmy指向的内容写入文件 字串6
finally 字串9
freemem(Pmy,count);
end;
end; 字串2
function Tmystream.read(var buffer;count:longint):longint;
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var
Pbu,Pmy,mykey:pchar; 字串9
i,mycount,enc:integer; 字串6
begin 字串5
getmem(Pmy,count);//为pmy分配内存 字串9
mykey:=pchar(key);//将key转换为pchar指针
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try 字串5
mycount:=inherited read(Pmy^,count); //将文件内容读入pmy指向内存区
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Pbu:=Pchar(@buffer);将buffer转换为pchar指针
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for i:=0 to mycount-1 do//将key的每个字符依次与pmy的每个字符循环相减，结果放入pbu指向的变量 字串2
begin 字串5
enc:=(ord(Pmy[i])-ord(mykey[(i mod length(key))])) mod 256; 字串8
Pbu[i]:=chr(enc);
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end;
finally
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freemem(Pmy,count); 字串9
end; 字串5
result:=mycount;
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end;
---- 完成定制文件流的编写后，便可在程序中应用，实现文件的读写加密，例程如下：
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unit Unit1; 字串4
interface
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uses Windows, Messages, SysUtils, Classes,Graphics, Controls, Forms, Dialogs,ExtCtrls, StdCtrls,unit2,unit3;
//unit2定义了Tmystream
//unit3定义了输入密码对话框form3
type 字串7
TForm1 = class(TForm) 字串5
Button1: TButton; 字串9
Button2: TButton; 字串4
Button3: TButton;
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OpenDialog1: TOpenDialog;
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SaveDialog1: TSaveDialog;
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Panel1: TPanel;
Panel2: TPanel;
Memo1: TMemo;
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Splitter1: TSplitter;
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Memo2: TMemo; 字串2
procedure Button2Click(Sender: TObject);
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procedure Button3Click(Sender: TObject); 字串5
procedure Button1Click(Sender: TObject); 字串5
private 字串1
{ Private declarations } 字串7
public 字串4
{ Public declarations } 字串5
end;
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var
Form1: TForm1; 字串9
implementation
{$R *.DFM} 字串4
procedure TForm1.Button2Click(Sender: TObject); //将选定的加密文件解开，读入memo2
var
encstr:Tmystream;
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begin
if opendialog1.Execute and (form3.showmodal=mrok) then
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begin
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encstr:=Tmystream.create(opendialog1.filename,fmopenread); 字串2
encstr.key:=form3.Edit1.Text;
try
memo2.lines.LoadFromStream(encstr); 字串2
finally
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encstr.Free; 字串3
end; 字串1
end; 字串2
end; 字串3
procedure TForm1.Button3Click(Sender: TObject); //将memo1中的内容加密，用指定文件名另存
var 字串3
encstr:Tmystream;
begin
if savedialog1.Execute and (form3.showmodal=mrok) then 字串2
begin
encstr:=Tmystream.create(savedialog1.filename,fmcreate); 字串7
encstr.key:=form3.Edit1.Text; 字串4
try 字串4
memo1.lines.SaveToStream(encstr); 字串4
finally 字串9
encstr.Free;
end;
end;
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end;
procedure TForm1.Button1Click(Sender: TObject); //将指定文件读入memo1 字串7
var 字串6
mystr:Tfilestream; 字串6
begin 字串6
if opendialog1.Execute then 字串3
begin 字串2
mystr:=Tfilestream.create(opendialog1.filename,fmopenread);
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try
memo1.lines.LoadFromStream(mystr);
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finally 字串1
mystr.Free; 字串5
end;
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end; 字串4
end; 字串9
end.