function TStringField.GetValue(var Value: string): Boolean;
begin
  Result := GetData(FIOBuffer);
  if Result then
    Value := TMarshal.ReadStringAsAnsi(TPtrWrapper.Create(FIOBuffer));
end;

class function TMarshal.ReadStringAsAnsi(Ptr: TPtrWrapper; Len: Integer): string;
begin
  Result := ReadStringAsAnsi(DefaultSystemCodePage, Ptr, Len);
end;

class function TMarshal.ReadStringAsAnsi(CodePage: Word; Ptr: TPtrWrapper; Len: Integer): string;
var
  dstLen: Integer;
begin
  if Len < 0 then
    if (Ptr.Value = nil) or (_PAnsiChr(Ptr.Value)^ = #0) then
      Exit('')
    else
      Len := _PCharLen(_PAnsiChr(Ptr.Value));

  dstLen := UnicodeFromLocaleChars(CodePage, 0, Pointer(Ptr.Value), Len + 1, nil, 0);
  if dstLen > 0 then
  begin
    SetLength(Result, dstLen - 1);
    if dstLen > 1 then // more than just null terminator
      UnicodeFromLocaleChars(CodePage, 0, Pointer(Ptr.Value), Len, Pointer(Result), dstLen);
  end
  else
  begin
    Result := '';
  end;
end;

var
  LBuf: TValueBuffer;
  LEnc: TEncoding;
begin
  SetLength(LBuf, 600);
  DBGrid1.SelectedField.GetData(LBuf);
  LEnc := TEncoding.GetEncoding(1251);
  try
    ShowMessage(LEnc.GetString(LBuf));
  finally
    LEnc.Free;
  end;
end;