Demonstration of the C# compiler using duck typing for the foreach keyword.

gistfile1.cs

namespace Samples
{
    using System;
    using System.Collections.Generic;

    public class CustomEnumerableDemoConsole
    {
        public static void Main(params string[] args)
        {
            CustomEnumerable enumerable = new CustomEnumerable("One", "Two", "Three");

            // "foreach" will work on any type that has a GetEnumerator() method, which returns
            // a type that has a Current property getter and a MoveNext() method.
            foreach (string item in enumerable)
            {
                Console.WriteLine(item);
            }
        }
    }

    // A custom enumerable which has a GetEnumerator() method, but does NOT implement IEnumerable.
    public class CustomEnumerable
    {
        // A custom enumerator which has a Current property and a MoveNext() method, but does NOT implement IEnumerator.
        public class CustomEnumerator
        {
            private readonly CustomEnumerable _enumerable;
            private int _index = -1;

            public CustomEnumerator(CustomEnumerable enumerable)
            {
                _enumerable = enumerable;
            }

            private IList<string> Items
            {
                get { return _enumerable._Items; }
            }

            public string Current
            {
                get { return _index >= 0 ? Items[_index] : null; }
            }

            public bool MoveNext()
            {
                if (_index < Items.Count-1)
                {
                    _index++;
                    return true;
                }
                return false;
            }
        }

        private IList<string> _Items;

        public CustomEnumerable(params string[] items)
        {
            _Items = new List<string>(items);
        }

        public CustomEnumerator GetEnumerator()
        {
            return new CustomEnumerator(this);
        }
    }
}