Extending the foreach loop by adding more elements inside the loop

Question

I have this loop:

        List<int> list = new List<int>();
        list.Add(1);
        list.Add(2);
        list.Add(3);

        int listCount = list.Count;
        for (int i = 0; i < listCount; i++) // Loop through List with for
        {
            if (i == 2)
            {
                list.Add(666);
                listCount++;
            }
            System.Diagnostics.Debug.Write(list[i]);                
        }

I am wondering how do I apply the same logic to foreach loop? (add more elements inside the foreach loop).

Something like this:

        List<int> list = new List<int>();
        list.Add(1);
        list.Add(2);
        list.Add(3);

        foreach (int i in list)
        {
            if (i == 2)
            {
                list.Add(666);
                //do something to refresh the foreach loop
            }
            System.Diagnostics.Debug.Write(list[i]);
        }

Is there any reason why you don't want to just use a `for` loop? — eddie_cat, Jul 21 '14 at 13:59
I fail to see a valid example for this, could you provide one? Adding elements inside a `foreach`-loop will in general not work due to the nature of enumeration. So I guess the "real" solution would be something different than adding elements inside the for-loop. — flindeberg, Jul 21 '14 at 14:13

Habib · Accepted Answer · 2014-07-21T14:03:45.503

I am wondering how do I apply the same logic to foreach loop?

You shouldn't and you can't. Collection can't be modified inside a foreach loop.

See: foreach, in (C# Reference)

If you need to add or remove items from the source collection, use a for loop.

One way to do it would be to make a copy of the List and then work over the original list like:

foreach (int i in list.ToList())
{
    if (i == 2)
    {
        list.Add(666);
        //do something to refresh the foreach loop
    }
    System.Diagnostics.Debug.Write(list[i]);
}

list.ToList would make a copy of the list and the foreach loop would work over it, thus saving you from the exception. But that is more like a hack, since you are not iterating the original list in the foreach loop.

score 1 · Answer 2 · answered Jul 21 '14 at 13:57

1

You can't. The foreach loop depends on the behavior of IEnumerable with the result being that the collection can not be modified during the iteration of the items. You will get a Runtime Exception every time you attempt to perform those types of modification to the collection.

answered Jul 21 '14 at 13:57

krillgar

12,596
6
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3

This behavior has nothing to do with foreach. It is caused by the enumerator implementation. – usr Jul 21 '14 at 13:58

score 1 · Answer 3 · answered Jul 21 '14 at 13:59

You could write yourself a custom enumerator or collection that allows you to modify the collection while enumerating. On the other hand, you probably should do something else entirely.

I'm not sure what you are trying to do, but this looks a lot better to me:

    List<int> list = new List<int>();
    list.Add(1);
    list.Add(2);
    list.Add(3);

    var outputList = new List<int>();
    foreach (int i in list)
    {
        if (i == 2)
        {
            outputList.Add(666);
            //do something to refresh the foreach loop
        }
        outputList.Add(list[i]);
    }

Just create a fresh list. Try not to mutate data structures. That's rather fragile. It is usually better to create new data from old data.

Ruchir Gupta · Answer 4 · 2014-07-21T14:05:50.113

1

Not exactly what you're trying to do but you could do something like this...

List<int> list = new List<int>();
list.Add(1);
list.Add(2);
list.Add(3);

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

foreach (int i in list)
{
    if (i == 2)
    {
        list2.Add(666);
    }
    System.Diagnostics.Debug.Write(list[i]);
    i++; // increment i
}

list = list2;

edited Jul 21 '14 at 14:05

answered Jul 21 '14 at 14:00

Ruchir Gupta

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Dmitry Bychenko · Answer 5 · 2014-07-21T14:21:09.230

If you're looking for a consize solution you can use Linq instead of either for loop or foreach loop (you can't change the collection within foreach loop):

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

  list.Add(1);
  list.Add(2);
  list.Add(3);

  // add required "666" items
  list.AddRange(Enumerable.Repeat(666, list.Count(x => x == 2)));

  // print out all items
  foreach(item in list)
    System.Diagnostics.Debug.Write(item);

score 0 · Answer 6 · answered Jul 21 '14 at 14:03

0

Yes and no with the 2 answers already given.

There is a way to modify collection (List actually) in a foreach loop, by using the following syntax, from LINQ :

List<int> list = new List<int>();
list.Add(1);
list.Add(2);
list.Add(3);
list.ForEach(elt => {
     if(elt == 2) {
         list.Add(666);
     }
      System.Diagnostics.Debug.Write(elt);
});

But be carefull... If you add "2" into your list within the condition, you'll go infinite :)

answered Jul 21 '14 at 14:03

FZysset

15
1
3

[`ForEach` is *not* `foreach`](http://stackoverflow.com/questions/225937/foreach-vs-somelist-foreach?answertab=votes#tab-top) – Habib Jul 21 '14 at 14:10
This will throw the same exception that you'd get for using a `foreach` loop. – Servy Jul 21 '14 at 14:17
@Habib That answer is dated. They have since changed `ForEach` to throw an exception if the collection is modified as of .NET 4.5. – Servy Jul 21 '14 at 14:18
@Servy, oh I didn't know that, I don't use `ForEach` and haven't find a place to really use it. – Habib Jul 21 '14 at 14:20

Extending the foreach loop by adding more elements inside the loop

6 Answers6