为什么TStringStream.Bytes与从'TStream.Read'获得的内容不同

问题描述

使用TStringStream，使用其 Bytes 属性的 bytes 与 bytes TStream.Read 提取的code>。如下所示：

With TStringStream, the bytes using its Bytes property differs from the bytes extracted using TStream.Read. As shown below:

1. 使用 TStream提取的个字节读代表正确的数据。


2. 字节使用其 Bytes 属性包含更多数据。

1. the bytes extracted using TStream.Read represents correct data.
2. the bytes using its Bytes property contains more data.

您能否评论可能的原因？

Could you help to comment about the possible reason? Thank you very much for your help!

PS：Delphi XE，Windows7。（看来Delphi 7中的TStringStream没有LoadFromFile或SaveToFile。）

PS: Delphi XE, Windows 7. (It seems TStringStream back in Delphi 7 doesn't have LoadFromFile or SaveToFile.)

PS：可以从SkyDrive下载示例文件：。 （Zlib压缩的图像文件。）。

PS: The sample files can be download from SkyDrive: REF_EncodedSample & REF_DecodedSample. (Zlib-compressed image file.).

procedure CompareBytes_2;
var
  ss_1: TStringStream;
  ss_2: TStringStream;
  sbytes_Read: TBytes;
  sbytes_Property: TBytes;
  len_sbytes_Read: Integer;
  len_sbytes_Property: Integer;
  filename: string;
begin
  filename := 'REF_EncodedSample';  // textual data
//  filename := 'REF_DecodedSample';  // non-textual data

  ss_1 := TStringStream.Create;
  ss_1.LoadFromFile(filename);
  ss_2 := TStringStream.Create;
  ss_2.LoadFromFile(filename);

  ss_1.SaveToFile(filename+ '_CopyByStringStream_1');
  ss_2.SaveToFile(filename+ '_CopyByStringStream_2');

  len_sbytes_Read := ss_1.Size;
  SetLength(sbytes_Read, len_sbytes_Read);
  ss_1.Read(sbytes_Read[0], len_sbytes_Read);

  sbytes_Property := ss_2.Bytes;

  ShowMessage(
    BoolToStr(
      Length(sbytes_Read) = Length(sbytes_Property),
      True));

  ShowMessage(
    BoolToStr(
      sbytes_Read[len_sbytes_Read - 1] = sbytes_Property[len_sbytes_Read - 1],
      True));

  ss_1.Free;
  ss_2.Free;
end;

推荐答案

字符串流状态：

假定缓冲区已分配为容纳更多的空间。实际需要。

Presumably the buffer has been allocated to hold more space than it actually needs. Only the first Size bytes of the buffer contain valid content.

此外，对ss_1的调用毫无意义。因为调用SetLength后Length（sbytes_Read）不变。从流中读取时，应使用ReadBuffer而不是Read。对于WriteBuffer也是如此。

Also, the call to ss_1.Read is a little pointless since Length(sbytes_Read) does not change after the call to SetLength. And when reading from a stream you are to use ReadBuffer rather than Read. Likewise for WriteBuffer.

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