Modifying collection inside foreach loop using IEnumerable Where()

Question

Although the iterator variable in a foreach loop is immutable, I am able to modify my collection in the example below if I use Where() to do the modification:

    static void Main(string[] args)
    {
        var list = new List<int> { 1, 2, 3, 4 };

        foreach (var listElement in list)
        {
            Console.WriteLine(listElement + " ");
            list = list.Where(x => x != listElement).ToList();
        }

        Console.WriteLine("Count: " + list.Count);
    }

Output:

1
2
3
4
Count: 0

Could someone please explain this behavior?

what was the expected output? – sujith karivelil Oct 26 '16 at 03:06 — sujith karivelil, Oct 26 '16 at 03:06
I expected this to have thrown an exception. – pmohandas Oct 26 '16 at 03:07 — pmohandas, Oct 26 '16 at 03:07

Lee Campbell · Accepted Answer · 2016-10-26T03:15:34.757

I believe that in the foreach (var listElement in list) you are getting the enumerator for the reference of list at that point in time.

In your loop you assign a new instance to the list variable, but the enumerator the loop is using keeps the reference to the original enumerator.

What you can not do is mutate the original instance. i.e. perform list.Remove(listElement) as this would be mutating the actual data you are trying to enumerate

It might help if the code is expanded to what the foreach loop is actually doing

static void Main(string[] args)
{
    var list = new List<int> { 1, 2, 3, 4 };
    var enumerator = list.GetEnumerator();
    while (enumerator.MoveNext())
    {
        var listElement = enumerator.Current;
        Console.WriteLine(listElement + " ");
        list = list.Where(x => x != listElement).ToList();
    }
    enumerator.Dispose();

    Console.WriteLine("Count: " + list.Count);
}

That makes sense. Thanks a lot! – pmohandas Oct 26 '16 at 03:16 — pmohandas, Oct 26 '16 at 03:16

score 2 · Answer 2 · answered Oct 26 '16 at 03:09

2

Code shown never "modifies collection" in common meaning of that phrase (add/remove elements from existing collection), instead it creates new collection. As result there is no "collection modified" exceptions that you probably expect.

answered Oct 26 '16 at 03:09

Alexei Levenkov

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score 0 · Answer 3 · answered Oct 26 '16 at 03:12

0

In fact collection is not being modified as you are not adding/removing any item from the collection. In terms of reference to list variable, I do agree with Lee i.e. the loop is keeping the reference to the original enumerator.

answered Oct 26 '16 at 03:12

Sami

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score 0 · Answer 4 · edited May 23 '17 at 10:27

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Because foreach uses an enumerator, and enumerators can't change the underlying collection, but can, however, change any objects referenced by an object in the collection. Refer the link. Also an interesting article worth reading to understand the behavior

edited May 23 '17 at 10:27

Community

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answered Oct 26 '16 at 03:24

Mukul Varshney

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Modifying collection inside foreach loop using IEnumerable Where()

4 Answers4