为什么TStringStream在转换为字符串时不会删除BOM？

Question

我们有一个库函数,如下所示:

class function TFileUtils.ReadTextStream(const AStream: TStream): string;
var
  StringStream: TStringStream;
begin
  StringStream := TStringStream.Create('', TEncoding.Unicode);
  try
    // This is WRONG since CopyFrom might rewind the stream (see Remys comment)
    StringStream.CopyFrom(AStream, AStream.Size - AStream.Position);
    Result := StringStream.DataString;
  finally
    StringStream.Free;
  end;
end;

当我检查函数返回的字符串时,第一个Char是(小端)BOM.

为什么TStringStream不会忽略BOM？

有一个更好的方法吗？我不需要向后兼容旧的Delphi版本,XE2的工作解决方案也没问题.

Answer 1

BOM必须来自源TStream,因为TStringStream不写BOM.如果要忽略BOM中的BOM(如果它存在于源中),则必须在复制数据之前手动执行,例如:

class function TFileUtils.ReadTextStream(const AStream: TStream): string;
var
  StreamPos, StreamSize: Int64;
  Buf: TBytes;
  NumBytes: Integer;
  Encoding: TEncoding;
begin
  Result := '';

  StreamPos := AStream.Position;
  StreamSize := AStream.Size - StreamPos;

  // Anything available to read?
  if StreamSize < 1 then Exit;

  // Read the first few bytes from the stream...
  SetLength(Buf, 4);
  NumBytes := AStream.Read(Buf[0], Length(Buf));
  if NumBytes < 1 then Exit;
  Inc(StreamPos, NumBytes);
  Dec(StreamSize, NumBytes);

  // Detect the BOM. If you know for a fact what the TStream data is encoded as, you can assign the Encoding variable to the appropriate TEncoding object and GetBufferEncoding() will check for that encoding's BOM only...
  SetLength(Buf, NumBytes);
  Encoding := nil;
  Dec(NumBytes, TEncoding.GetBufferEncoding(Buf, Encoding));

  // If any non-BOM bytes were read than rewind the stream back to that position...
  if NumBytes > 0 then
  begin
    AStream.Seek(-NumBytes, soCurrent);
    Dec(StreamPos, NumBytes);
    Inc(StreamSize, NumBytes);
  end else
  begin
    // Anything left to read after the BOM?
    if StreamSize < 1 then Exit;
  end;

  // Now read and decode whatever is left in the stream...
  StringStream := TStringStream.Create('', Encoding);
  try
    StringStream.CopyFrom(AStream, StreamSize);
    Result := StringStream.DataString;
  finally
    StringStream.Free;
  end;
end;