Why List.Remove() method removing entries from the calling method's list?

Question

Pardon me if its a very foolish question. But I want to know why this is happening.

My code is

    static void Main(string[] args)
        {
            List<int> lst = new List<int>();
            for (int i = 0; i< 10; i++)
            {
                lst.Add(i);
            }

            List<int> lstRet = lst;
            List<int> lstRet2 = lstReturn(lstRet);
         }

        static List<int> lstReturn(List<int> lst)
        {
            List<int> NewList = lst;
            NewList.Remove(3);
            NewList.Remove(6);
            NewList.Remove(9);

            return NewList;
        }

The problem is after the method lstReturn(lstRet); is getting called, both the list Values values are same, as if lst and lstRet2 both are identical List.

Why it is like that. Can anybody please explain? Is only one copy of List is created, if yes, why?

I am again saying if its a duplicate, pardon me, I searched but didn't get any satisfactory answer.

Thanks

Edit: The question was not a duplicate, the solution lies elsewhere, may be the fundamental knowledge is same. Instead passing the same list lst to lstRet, if its like

List<int> lstRet = New List<int>(lst)

this solved the problem, Thanks to all of you for such a quick response.

The list is copied by reference. So you are modifying the same list. — Lews Therin, Mar 14 '15 at 19:59
Why down voted, I told that I didn't have idea why its happening. Instead of down voting if somebody explains would be good for me and for others as well, not everybody is Einstein. — Debhere, Mar 14 '15 at 20:02
And I don't know why the answers *are* downvoted. Considering they are correct. — Lews Therin, Mar 14 '15 at 20:02
I didn't downvoted any answers, all are giving some kind of light to my little knowledge. Thanks to all of you. — Debhere, Mar 14 '15 at 20:03
@LewsTherin There's this practice on SO when people use downvoting to prod others to delete answers that the downvoters dislike for any reason. I do not think it's a good practice, but it exists nevertheless. — Sergey Kalinichenko, Mar 14 '15 at 20:07
@dasblinkenlight Yeah I've noticed. And consider it distasteful. But I promise you that although mad.. I am innocent in this. — Lews Therin, Mar 14 '15 at 20:12

score 3 · Accepted Answer · answered Mar 14 '15 at 19:59

3

The problem is with this line:

List<int> lstRet = lst;

It does not do what you want it to do - instead of copying the list, it copies the reference. You have only one list, from which the items get removed.

If you wish to make a copy, replace the line above with

List<int> lstRet = new List<int>(lst);

answered Mar 14 '15 at 19:59

Sergey Kalinichenko

714,442
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Thanks it really helps me. This is what I actually missed, Thanks for the insight. – Debhere Mar 14 '15 at 20:08

score 1 · Answer 2 · answered Mar 14 '15 at 19:59

1

They are the same list. You are assigning the return value of lstReturn to the variable lstRet2:

List<int> lstRet2 = lstReturn(lstRet);

and the function lstReturn returns the list that was passed in:

static List<int> lstReturn(List<int> lst)
    {
        List<int> NewList = lst; // NewList now is the list that was passed in

        // snip...

        return NewList;
    }

answered Mar 14 '15 at 19:59

Chris Shain

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Your answer and explanation is beautiful but I can mark only one answer as accepted. @Chris – Debhere Mar 14 '15 at 20:19

Why List.Remove() method removing entries from the calling method's list?

2 Answers2