public File(OnDiskMemoryMappedFileCollection owner, uint index, string fileName)
{
_owner = owner;
_index = index;
_fileName = fileName;
_stream = new FileStream(fileName, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.Read, 1, FileOptions.None);
_trueLength = _stream.Length;
_writer = new Writer(this);
}