Prune method.

Prune.cs

private void Prune()
{
  DateTime expirationTime = DateTime.Now - _lifetime;
  while (true)
  {
    var node = _list.Last;
    if (node == null || node.Value.Value.TimeStamp > expirationTime)
    {
      return;
    }
    _list.RemoveLast();
    _dict.Remove(node.Value.Key);
  }
}