c# - Can't add/remove items from a collection while foreach is iterating over it

Question

If I make my own implementation of IEnumerator interface, then I am able ( inside foreach statement )to add or remove items from a albumsList without generating an exception.But if foreach statement uses IEnumerator supplied by albumsList, then trying to add/delete ( inside the foreach )items from albumsList will result in exception:

class Program
{
    static void Main(string[] args)
    {

        string[] rockAlbums = { "rock", "roll", "rain dogs" };
        ArrayList albumsList = new ArrayList(rockAlbums);
        AlbumsCollection ac = new AlbumsCollection(albumsList);
        foreach (string item in ac)
        {
            Console.WriteLine(item);
            albumsList.Remove(item);  //works

        }

        foreach (string item in albumsList)
        {
            albumsList.Remove(item); //exception
        }



    }

    class MyEnumerator : IEnumerator
    {
        ArrayList table;
        int _current = -1;

        public Object Current
        {
            get
            {
                return table[_current];
            }
        }

        public bool MoveNext()
        {
            if (_current + 1 < table.Count)
            {
                _current++;
                return true;
            }
            else
                return false;
        }

        public void Reset()
        {
            _current = -1;
        }

        public MyEnumerator(ArrayList albums)
        {
            this.table = albums;
        }

    }

    class AlbumsCollection : IEnumerable
    {
        public ArrayList albums;

        public IEnumerator GetEnumerator()
        {
            return new MyEnumerator(this.albums);
        }

        public AlbumsCollection(ArrayList albums)
        {
            this.albums = albums;
        }
    }

}

a) I assume code that throws exception ( when using IEnumerator implementation A supplied by albumsList ) is located inside A?

b) If I want to be able to add/remove items from a collection ( while foreach is iterating over it), will I always need to provide my own implementation of IEnumerator interface, or can albumsList be set to allow adding/removing items?

thank you

Answer 1

Easiest way is to either reverse through the items like for(int i = items.Count-1; i >=0; i--), or loop once, gather all the items to remove in a list, then loop through the items to remove, removing them from the original list.