FileOpen, FileRead, FileWrite

Question

How to properly work with FileRead, FileWrite, buffers (or with TFileStream).

I need to read a whole text file to a String, then to write back a String to this file

Answer 1

Added requested example of working with FileXXX function family too. Note on using RawByteString and file I/O - it is perfectly valid. Another note: for brevity in the low-level code i omitted some error-checking assertions for less likely occurring errors. Caveat emptor!

const
  FileName = 'Unit14.pas';  // this program is a stuntmaster,
                            // reads and writes its own source

procedure TForm14.FormClick(Sender: TObject);
var
  Stream: TFileStream;
  Buffer: RawByteString;
begin
  Stream := TFileStream.Create(FileName, fmOpenReadWrite or fmShareExclusive);
  // read entire file into string buffer
  SetLength(Buffer, Stream.Size);
  Stream.ReadBuffer(Buffer[1], Stream.Size);

  // do something with string
  OutputDebugString(PChar(Format('Buffer = "%s"', [Buffer])));

  // prepare to write
  Stream.Position := 0;   // rewind file pointer
  Stream.Size := 0;       // truncate the file

  // write entire string into the file
  Stream.WriteBuffer(Buffer[1], Length(Buffer));

  Stream.Free;
end;

// on right click - do exactly the same but using low-level FileXXX calls
procedure TForm14.FormContextPopup(Sender: TObject; MousePos: TPoint; var
    Handled: Boolean);
var
  Handle: Integer;
  Size: Cardinal;
  Buffer: RawByteString;
  Transferred: Integer;
begin
  Handle := FileOpen(FileName, fmOpenReadWrite or fmShareExclusive);
  Assert(Handle >= 0);

  // read entire file into string buffer
  Size := GetFileSize(Handle, nil);
  Assert(Size <> INVALID_FILE_SIZE);
  SetLength(Buffer, Size);
  Transferred := FileRead(Handle, Buffer[1], Size);
  Assert(not (Transferred < Size));

  // do something with string
  OutputDebugString(PChar(Format('Buffer = "%s"', [Buffer])));

  // prepare to write
  FileSeek(Handle, 0, 0); // rewind file pointer
  SetEndOfFile(Handle);   // truncate the file

  // write entire string into the file
  Transferred := FileWrite(Handle, Buffer[1], Length(Buffer));
  Assert(not (Transferred < Length(Buffer)));

  FileClose(Handle);
end;