Stream objects to a file using TFileStream

Why does this code not work?

I am writing an application that has ability to save and load its own files and need to know how to stream objects to a file using FileStream.

procedure TForm1.btnSaveClick(Sender: TObject);
var
  fs: 开发者_JAVA技巧TFileStream;
begin
  fs := TFileStream.Create('c:\temp\a.my', fmCreate);
  try
    fs.WriteBuffer(Image1.Picture.Graphic, SizeOf(TGraphic));
  finally
    fs.Free;
  end;

  ShowMessage('ok');

  Image1.Picture.Graphic := nil;
end;

procedure TForm1.btnLoadClick(Sender: TObject);
var
  fs: TFileStream;
  g:  TGraphic;
begin
  fs := TFileStream.Create('c:\temp\a.my', fmOpenRead);
  try
    fs.ReadBuffer(g, SizeOf(TGraphic));

    Image1.Picture.Graphic := g;
  finally
    fs.Free;
  end;

  ShowMessage('ok');

end;

EDIT 1: Found the way to do it, but need some more help:

procedure TForm1.btnSaveClick(Sender: TObject);
var
  fs: TFileStream;
  s:  TMemoryStream;
  buf: TBytes;
begin
  fs := TFileStream.Create('c:\temp\a.my', fmCreate);
  s  := TMemoryStream.Create;
  try
    Image1.Picture.Graphic.SaveToStream(s);

    SetLength(buf, s.Size);
    s.Position := 0;
    s.ReadBuffer(buf[0], s.Size);

    //fs.WriteBuffer(, SizeOf(Integer));  <-here how do I save an integer which represents the size of the buffer? (so that when reading back i read this first.) 

    fs.WriteBuffer(buf[0], s.Size);
  finally
    s.Free;
    fs.Free;
  end;

  ShowMessage('ok');

  Image1.Picture.Graphic := nil;
end;

What you have done there is stream the reference, i.e. a pointer. What you need to stream is the contents. You can that with SaveToFile and LoadFromFile.

Regarding your update, assign s.Size to a local variable of type Integer and then use WriteBuffer to save it. In reverse, use ReadBuffer to read into a local variable.

If I were you I would write direct to the file and avoid the memory streak. Use the Position property of TStream to seek around the file. So write 0 for then length, write the graphic, seek back to the beginning and write the true length accounting for the 4 bytes of the length.