type
  TTextFormat=(tfAnsi,tfUnicode,tfUnicodeBigEndian,tfUtf8);
const
  TextFormatFlag:array[tfAnsi..tfUtf8] of word=($0000,$FFFE,$FEFF,$EFBB);　

function WordLoHiExchange(w:Word):Word;register;
asm
  XCHG AL, AH
end;

{ TextFormat返回文本编码类型，sText未经处理的文本 }
procedure ReadTextFile(const FileName: string;
  var TextFormat: TTextFormat; var sText:string);
var
  w:Word;
  b:Byte;
begin
  with TFileStream.Create(FileName,fmOpenRead or fmShareDenyNone) do
  try
    Read(w,2);
    w:=WordLoHiExchange(w);//因为是以Word数据类型读取，故高低字节互换
    if w = TextFormatFlag[tfUnicode] then
      TextFormat:= tfUnicode
    else if w = TextFormatFlag[tfUnicodeBigEndian] then
      TextFormat:= tfUnicodeBigEndian
    else if w = TextFormatFlag[tfUtf8] then
    begin
      Read(b,1);//这里要注意一下，UFT-8必须要跳过三个字节。
      TextFormat:=tfUtf8;
    end else
    begin
      TextFormat:=tfANSI;
      Position:=0;
    end;
    SetLength(sText,Size-Position);
    ReadBuffer(sText[1],Size-Position);
  finally
    Free;
  end;
end;