Exercise solution:
Infinite Collections of Integers

using System;
using System.Collections.Generic;
using System.Collections;
using System.Linq;                 // only because we use Take in Main

    public class IntegersFrom : IEnumerable<long>
    {
        private long low;

        public IntegersFrom(long low)
        {
            this.low = low;
        }

        public long LowerLimit
        {
            get { return low; }
        }

        private class IntegerEnumerator : IEnumerator<long>
        {
            private IntegersFrom intfr;
            private long curint;

            public IntegerEnumerator(IntegersFrom intfr)
            {
                this.intfr = intfr;
            }

            public long Current { get { return curint; } }

            object IEnumerator.Current { get { return curint; } }  // !!

            public bool MoveNext() { curint++; return true; }

            public void Reset() { curint = intfr.low; }

            void IDisposable.Dispose(){}  // !!
        }

        public IEnumerator<long> GetEnumerator()
        {
            return new IntegerEnumerator(this);
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }


    }

    public class IntegersFromAlternative : IEnumerable<long>
    {
        private long low;

        public IntegersFromAlternative(long low)
        {
            this.low = low;
        }

        public long LowerLimit
        {
            get { return low; }
        }

        public IEnumerator<long> GetEnumerator(){
            for(long i = low; true; i++)
                yield return i;
        }

        IEnumerator IEnumerable.GetEnumerator()  // !!
        {
            return GetEnumerator();
        }
     

    }

    internal static class Extensions
    {
        public static IEnumerable<long> AdInfinitum(this long from)
        {
            for (long i = from; true; i++)
                yield return i;
        }

        public static IEnumerable<long> Filter(this IEnumerable<long> source, Func<long,bool> pred)
        {
            foreach (long i in source)
                if (pred(i)) yield return i;
        }

        public static IEnumerable<long> Add(this IEnumerable<long> source, IEnumerable<long> other)
        {
            IEnumerator<long> sourceEnum = source.GetEnumerator();
            IEnumerator<long> otherEnum = other.GetEnumerator();

            while (true)
            {
                yield return sourceEnum.Current + otherEnum.Current;
                sourceEnum.MoveNext();
                otherEnum.MoveNext();
            }
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var itf1 = new IntegersFrom(1);
            var itf2 = new IntegersFromAlternative(1);

            // foreach (long i in itf1) { Console.WriteLine(i); }

            IEnumerable<long> natNums = 1L.AdInfinitum();

            // foreach (long i in natNums) { Console.WriteLine(i); }

            //foreach (long i in natNums.Filter(n => n % 2 == 0))
            //{
            //    Console.WriteLine(i);
            //}

            //var even100 = natNums.Filter(n => n % 2 == 0).Take(100);
            //foreach (long i in even100) Console.WriteLine(i);

            var evenNumbers = natNums.Add(natNums).Take(100);
            foreach (long i in evenNumbers) Console.WriteLine(i);

            Console.ReadLine();
        }
    }