Storing iterator state in a collection can have unexpected consequences.

UnexpectedLoop.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace UnexpectedLoop
{
    class Program
    {
        static void Main(string[] args)
        {
            // MyCollection stores its iterator state in the collection, so does not work as expected.
            Console.WriteLine("MyCollection:");
            MyCollection mine = new MyCollection();
            PrintValues(mine);

            // List<int> stores its iterator state in a separate class and
            // creates a new instance of that class for each GetEnumerator call.
            Console.WriteLine("List<int>:");
            List<int> theirs = new List<int>(new int[] { 7 });
            PrintValues(theirs);
        }

        static void PrintValues(IEnumerable<int> values)
        {
            bool numberOfValuesDisplayed = false;
            foreach (int value in values)
            {
                // Use LINQ to enumerate the collection again!
                if (!numberOfValuesDisplayed)
                {
                    Console.WriteLine("The collection has this many values: {0}", values.Count());
                    numberOfValuesDisplayed = true;
                }

                Console.WriteLine("{0}", value);
            }
        }

        // Based on the example IEnumerator implementation on MSDN:
        // http://msdn.microsoft.com/en-us/library/system.collections.ienumerator.aspx
        class MyCollection : IEnumerable<int>, IEnumerator<int>
        {
            int[] values = new int[1]{7};
            int position = -1;

            public IEnumerator<int> GetEnumerator()
            {
                return this;
            }

            System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
            {
                return this;
            }

            public int Current
            {
                get { return values[position]; }
            }

            public void Dispose()
            {
            }

            object System.Collections.IEnumerator.Current
            {
                get { return (object)values[position]; }
            }

            public bool MoveNext()
            {
                position++;
                return position < values.Length;
            }

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