using System;

using System.Collections;

using System.Collections.Generic;

public class Program

{

    public static int a = 1;

    public static int c = 1;

    public static void Main()

    {

        var sequence = GenerateSequence1();

        a = 2;

        c = 2;

        foreach(var num in GenerateSequence1()) 

            Console.WriteLine(num);     

    }

    public static IEnumerable<int> GenerateSequence1()

    {

        return new Enumerable();

    }           

}

​

public class Enumerable : IEnumerable<int>

{

    public IEnumerator<int> GetEnumerator() => new Enumerator();

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();

}

​

public class Enumerator : IEnumerator<int>

{

    private int state;

    public int Current { get; private set; }

    public bool MoveNext()

    {

        switch (state)

        {

            case 0:

                Current = Program.a;

                state = 1;

                return true;

            case 1:

                Current = 3;

                state = 2;

                return true;

            case 2:

                if (Program.c == 2)

                {

                    Current = 4;

                    state = 3;

                    return true;

                }

                else

                {

                    state = 3;

                    return false;

                }

            case 3:

            default:

                return false;

        }

    }

    object IEnumerator.Current => Current;

    public void Dispose() { }

    public void Reset() => throw new NotSupportedException();

}