using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Collections;
 
namespace mycs
{
  class Program
  {
    static void Main(string[] args)
    {
      charlist mycharlist = new charlist("hello world");
      foreach (var c in mycharlist)
      {
        Console.Write(c);
      }
     Console.ReadLine();
    }
  }
 
  class charlist : IEnumerable
  {
    public string TargetStr { get; set; }
 
    public charlist(string str)
    {
      this.TargetStr = str;
    }
    public IEnumerator GetEnumerator()
    {
      //c# 1.0
      return new CharIterator(this.TargetStr);
      //c# 2.0
      /*
      for (int index = this.TargetStr.Length; index > 0;index-- )
      {
        yield return this.TargetStr[index - 1];
      }
       */
    }
  }
  class CharIterator : IEnumerator
  {
    public string TargetStr { get; set; }
    public int position { get; set; }
 
    public CharIterator(string targetStr)
    {
      this.TargetStr = targetStr;
      this.position = this.TargetStr.Length;
    }
    public object Current
    {
      get
      {
        if (this.position==-1||this.position==this.TargetStr.Length)
        {
          throw new InvalidOperationException();
        }
        return this.TargetStr[this.position];
      }
    }
    public bool MoveNext()
    {
      if (this.position!=-1)
      {
        this.position--;
      }
      return this.position > -1;
    }
    public void Reset()
    {
      this.position = this.TargetStr.Length;
    }
  }
}