What is the quickest way to create an independent duplicate of a list (not the objects in it)?

Question

Say I have a ListA. I want to make a duplicate ListB so when I use ListB.Remove(SomeItem); it would not affect the listA.

The list items should refer to the same objects.

Since this is being used in a loop it is important to use least resource demanding approach. But I don't know how to do this in most efficient way.

I think the LINQ library supports a function that is called Copy(). But basically you can just create new List () and then iteratively add each element in ListA to the new list. Or just juse newList.AddRange(listA) — Glubus, Jan 25 '16 at 11:02
Possible duplicate of [Deep copy of List](http://stackoverflow.com/questions/4226747/deep-copy-of-listt) — MakePeaceGreatAgain, Jan 25 '16 at 11:03
What should happen when you modify `someItem` instead of deleting it? Should the change be reflected in both lists? — MakePeaceGreatAgain, Jan 25 '16 at 11:04
It's a duplicate question... Anyways. Why don't you create two lists initially and add items to list 'B' along with list 'A'? — , Jan 25 '16 at 11:04

Matthew Watson · Answer 1 · 2016-01-25T11:32:00.887

You want a shallow clone, which you can do simply using this List<T> constructor:

var shallowClone = new List(originalList);

Note that the implementation of this constructor checks if originalList supports the Count property, and if it does it uses it to pre-size the list to avoid unnecessary memory allocations. This makes it pretty optimal:

_items = new T[count];
c.CopyTo(_items, 0);
_size = count;

However, I've looked at the implementation of Enumerable<T>.ToList() and all it does is call the List<T> constructor above.

So this code does pretty much exactly the same thing (and will run at the same speed):

var shallowClone = originalList.ToList();

The easiest and simplest way! – Gian Paolo Jan 25 '16 at 11:10 — Gian Paolo, Jan 25 '16 at 11:10

MakePeaceGreatAgain · Answer 2 · 2016-01-25T11:32:57.880

3

You can simply call ListB.ToList() which will create a new list from ListB sharing the same elements. Changes to the element-SET of ListB are not reflected in A, however if you change any element it is reflected in both lists.

edited Jan 25 '16 at 11:32

answered Jan 25 '16 at 11:04

MakePeaceGreatAgain

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Thanks Him; This is a very good answer. But there might be a quicker way. – Saeid Jan 25 '16 at 11:22
I dount there is a QUICKER way as all this implementation does is iterating the first list and creating a new list from it. However there is a neater way as suggested by Matthew. – MakePeaceGreatAgain Jan 25 '16 at 11:26
You don't need the `.Select()` btw. With the `Select()` this will be significantly slower. – Matthew Watson Jan 25 '16 at 11:30
1

@MatthewWatson I wasn´t sure if `ToList` will eventually cast the instance to `list` if it already is one. Yiu may be right however. – MakePeaceGreatAgain Jan 25 '16 at 11:32
1

This is actually now one character shorter than using the `List` constructor directly. :) – Matthew Watson Jan 25 '16 at 11:33

score 2 · Answer 3 · answered Jan 25 '16 at 11:06

2

In your case u can use List Generic Method. Add this method in your class.

public static List<T> Clone<T>(this List<T> myList)
{
    var newList = new List<T>(myList.Capacity);
    newList.AddRange(myList);
    return newList;
}

answered Jan 25 '16 at 11:06

rdn87

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score 1 · Accepted Answer · answered Jan 25 '16 at 12:35

Interestingly, no one seems to be looking for a specialized List<T> method (which by definition should have an optimal implementation), and there is one called GetRange which according to the documentation

Creates a shallow copy of a range of elements in the source List<T>.

So creating a shallow copy of the whole source list would be something like this

var shallowClone = originalList.GetRange(0, originalList.Count);

P.S. Of course the difference in performance from the most concise

var shallowClone = originalList.ToList();

and not so concise

var shallowClone = new List<MyLongListItemType>(originalList);

will be negligible.

So I would personally prefer ToListversion, and at the same time will remember to not call ToList in my code without a need (which I've seen in many places) because of the copy behavior of that method.

What is the quickest way to create an independent duplicate of a list (not the objects in it)?

4 Answers4