using System;
using System.Collections;
using System.Collections.Generic;

class Program {
    // *** before: ***
    // to be usable in foreach a type had to carry an enumerator written by
    // hand: a state field, MoveNext, Current and Reset
    class EvenNumbersOld : IEnumerable {
        int bound;
        public EvenNumbersOld(int bound) { this.bound = bound; }
        public IEnumerator GetEnumerator() { return new Enumerator(bound); }

        class Enumerator : IEnumerator {
            int bound, current;
            public Enumerator(int bound) { this.bound = bound; this.current =
                          0; }
            public bool MoveNext() { current += 2; return current <= bound; }
            public object Current { get { return current; } }
            public void Reset() { current = 0; }
        }
    }

    // *** in version 2.0: ***
    // yield return writes that state machine for us
    static IEnumerable<int> EvenNumbers(int bound) {
        for (int n = 2; n <= bound; n += 2)
            yield return n;               // the method pauses here
    }

    static IEnumerable<int> FirstThree(IEnumerable<int> source) {
        int taken = 0;
        foreach (int n in source) {
            if (taken == 3) yield break;  // stop early
            taken++;
            yield return n;
        }
    }

    static void Main() {
        foreach (int n in new EvenNumbersOld(10)) Console.Write(n + " ");
        Console.WriteLine();

        foreach (int n in EvenNumbers(10)) Console.Write(n + " ");
        Console.WriteLine();

        // the values are produced one by one, so an endless source is fine
        foreach (int n in FirstThree(EvenNumbers(int.MaxValue))) Console.Write(
                          n + " ");
        Console.WriteLine();
    }
}