How to preserve lists already created before using List.Clear()

Question

When I add data to a List, then append that list to another list, then use List.Clear() on the original list, it empties everything and the lists already appended aren't preserved.

Here's an example of what I'm talking about. Lets say I make 2 lists like this:

List<int> list = new List<int>();
List<List<int>> list2 = new List<List<int>>();

for(int i=0;i<10;i++){

  for(int i2=0;i2<10;i2++){
    list.Add(i2);
  }
  list2.Add(list);
  list.Clear();

}

When I run list.Clear() it clears out all of the pre-existing lists I already appended to list2

I know a work-around would be to re-arrange the code like this:

List<List<int>> list2 = new List<List<int>>();

for(int i=0;i<10;i++){

  List<int> list = new List<int>(); //<- moved inside the for

  for(int i2=0;i2<10;i2++){
    list.Add(i2);
  }
  list2.Add(list);

}

But is this normal behavior? Is it possible to preserve the pre-appended lists?

Answer 1

Classes are passed by reference and List<T> is a class. Accordingly, when you clear list it also clears the data passed by reference in list2. See List - do I pass objects or references? for more details.

This should solve the issue shown in your example and create a new copy of list to add to list2 before clearing list:

List<int> list = new List<int>();
List<List<int>> list2 = new List<List<int>>();

for(int i=0;i<10;i++){

  for(int i2=0;i2<10;i2++){
    list.Add(i2);
  }
  list2.Add(list.ToList()); //modified to create a new List when adding
  list.Clear();

}

See also Jon Skeet's Parameter passing in C# for some broader information on how parameters are passed in C#, especially relating to reference types.