List thread safe?

Question

Can the following be considered thread safe due to the atomic operation appearance of the code. My main concern is if the lists needs to be re-sized it becomes non-thread safe during the re-sizing.

List<int> list = new List<int>(10);

public List<int> GetList()
{
  var temp = list;
  list = new List<int>(10);
  return temp;
}

TimerElapsed(int number)
{
  list.Add(number);
}

Answer 1

No. List<T> is explicitly documented not to be thread-safe:

It is safe to perform multiple read operations on a List, but issues can occur if the collection is modified while it’s being read. To ensure thread safety, lock the collection during a read or write operation. To enable a collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization. For collections with built-in synchronization, see the classes in the System.Collections.Concurrent namespace. For an inherently thread–safe alternative, see the ImmutableList class.