Question

In the following code, is the Select() method smart enough to keep the size of the list somewhere internally for the ToArray() method to be cheap?

List<Thing> bigList = someBigList;
var bigArray = bigList.Select(t => t.SomeField).ToArray();

Answer 1

That’s easy to check, without looking at the implementation. Just create a class that implements IList<T>, and put a trace in the Count property:

    class MyList<T> : IList<T>
    {
        private readonly IList<T> _list = new List<T>();
        public IEnumerator<T> GetEnumerator()
        {
            return _list.GetEnumerator();
        }

        public void Add(T item)
        {
            _list.Add(item);
        }

        public void Clear()
        {
            _list.Clear();
        }

        public bool Contains(T item)
        {
            return _list.Contains(item);
        }

        public void CopyTo(T[] array, int arrayIndex)
        {
            _list.CopyTo(array, arrayIndex);
        }

        public bool Remove(T item)
        {
            return _list.Remove(item);
        }

        public int Count
        {
            get
            {
                Console.WriteLine ("Count accessed");
                return _list.Count;
            }
        }

        public bool IsReadOnly
        {
            get { return _list.IsReadOnly; }
        }

        public int IndexOf(T item)
        {
            return _list.IndexOf(item);
        }

        public void Insert(int index, T item)
        {
            _list.Insert(index, item);
        }

        public void RemoveAt(int index)
        {
            _list.RemoveAt(index);
        }

        public T this[int index]
        {
            get { return _list[index]; }
            set { _list[index] = value; }
        }

        #region Implementation of IEnumerable

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }

        #endregion
    }

If the Count property is accessed, this code should print “Count accessed”:

var list = new MyList<int> { 1, 2, 3 };
var array = list.Select(x => x).ToArray();

But it doesn’t print anything, so no, it doesn’t keep track of the count. Of course there could be an optimization specific to List<T>, but it seems unlikely…