Question

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

1. the bytes extracted using TStream.Read represents correct data.
2. the bytes using its Bytes property contains more data. (the last byte of correct bytes is different from that of wrong bytes)

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

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

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;

Answer 1

The string stream documentation states:

The Bytes property returns the buffer in which the data is stored. Use the Size property to find the actual amount of data in the buffer.

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.

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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.