using System;

using System.Collections;

class PrimeNums : IEnumerable

{

    class Enumerator : IEnumerator

    {

        int _idx = 0;

        int _current;

        int[] _pmArr = new int[] { 2, 3, 5, 7 };

​

        public object Current => _current;

        public bool MoveNext()

        {

            if (_pmArr.Length - 1 < _idx)

            {

                return false;

            }

            _current = _pmArr[_idx];

            _idx++;

            return true;

        }

        public void Reset()

        {

            _idx = 0;

        }

    }

    public IEnumerator GetEnumerator()

    {

        return new Enumerator();

    }

}

​

class Program

{

    public static void Main()

    {

        Console.WriteLine("---DoForeach");

        DoForeach();

        Console.WriteLine("---DoEnumerate");

        DoEnumerate();

    }

    private static void DoForeach()

    {

        PrimeNums primeNums = new PrimeNums();

​

        foreach (var item in primeNums)

        {

            Console.WriteLine(item);

        }

    }

    private static void DoEnumerate()

    {

        PrimeNums primeNums = new PrimeNums();

​

        var enumerator = primeNums.GetEnumerator();

        while (enumerator.MoveNext())

        {

            var item = enumerator.Current;

            Console.WriteLine(item);

        }

    }

}

​