copying a list to a new list - More efficient & Best Practice

Question

What is the appropriate way of copying a list to a new list? And What is the most efficient way of copying a list to a new list?

By efficient, not code efficiency, more in the behind the scenes framework sense.

List<String>List2 = List.ToList();

Or:

List<String>List2 = new List<String>();
foreach (string item in List)
{
 List2.Add(item);
}

Update:

What about more efficient IL code?

Answer 1

Given that List<T> has an IEnumerable<T> constructor, I would prefer this form:

List<string> newList = new List<string>(otherList);

Edit

And as Ondrej points out in the decompiled code below, the constructor of List<T> preallocates the size of the array and copies the contents over. This is going to be much quicker than creating a new list and then iterating over the other list adding items individually, especially as in your 2nd example you're not specifying how many items to preallocate.

Answer 2

What ToList does (shortened):

public static List<TSource> ToList<TSource>(this IEnumerable<TSource> source)
{
    return new List<TSource>(source);
}

What List ctor does (shortened):

public List(IEnumerable<T> collection)
{
    ICollection<T> collection2 = collection as ICollection<T>;
    int count = collection2.Count;
    this._items = new T[count];
    collection2.CopyTo(this._items, 0);
    this._size = count;
}

So the ToList() is much more efficient - it does allocate the space first and then copy all items in one step.