using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var evilList = new EvilList<int>(new[] { 1, 2, 3, 4, 5});

​

        Console.WriteLine(evilList.First()); //using indexer, outputs `1`

        Console.WriteLine(evilList.Where(i => true).First()); //using IEnumerable.GetEnumerator, outputs `5`

​

        foreach(var item in evilList) //using IEnumerable, outputs `5, 4, 3, 2, 1`

            Console.WriteLine(item);

    }

    public class EvilList<T> : IList<T>

    {

        private readonly List<T> Items;

​

        public int Count => Items.Count;

​

        public T this[int index] { get => Items[index]; set => Items[index] = value; }

​

        public EvilList(IEnumerable<T> items)

        {

            this.Items = new List<T>(items);

        }

​

        public IEnumerator<T> GetEnumerator()

        {

            //EVIL enumerator that enumerates in reverse order

            for (int i = Items.Count - 1; i >= 0; i--)

            {

                yield return Items[i];

            }

        }

​

        IEnumerator IEnumerable.GetEnumerator()

        {

            return GetEnumerator();

        }

​

        //rest of IList<T> implementation that we don't care about      

        public bool IsReadOnly => throw new NotImplementedException();

​

        public int IndexOf(T item)

        {

            throw new NotImplementedException();

        }

​

        public void Insert(int index, T item)

        {

            throw new NotImplementedException();

        }

​

        public void RemoveAt(int index)

        {

            throw new NotImplementedException();

        }

​

        public void Add(T item)

        {

            throw new NotImplementedException();

        }

​

        public void Clear()

        {

            throw new NotImplementedException();

        }

​

        public bool Contains(T item)

        {

            throw new NotImplementedException();

        }

​

        public void CopyTo(T[] array, int arrayIndex)

        {

            throw new NotImplementedException();

        }

​

        public bool Remove(T item)

        {

            throw new NotImplementedException();

        }

    }

}