C# Online Compiler | .NET Fiddle

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);
        }
    }
}

        var enumerator = primeNums.GetEnumerator();
 
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()
        {