ListExtension.cs

public static class ListExtension
{
  public static void Add<T>(this List<T> list, T value, out List<T> newInstance)
  {
    if (list == null) list = new List<T>();
    list?.Add(value);
    newInstance = list;
  }
}

[Test]
public void ListAddTest()
{
  List<string> list = null;
  list.Add("test",out list);

  Assert.AreEqual(1,list.Count);
}