void CloseFile()
{
_currentFile.Dispose();
_currentFile = null;
_limitOfCurrentFile = null;
}
void OpenFile(DateTimeOffset timeStamp)
{
var limit = timeStamp.Date.AddDays(1);
var path = string.Format(_pathFormat, timeStamp.Date.ToString("yyyy-MM-dd"));
_currentFile = new FileSink(path, _textFormatter);
_limitOfCurrentFile = limit;
}