working pattern of yield return

Question

When i have a code block

static void Main()
{

  foreach (int i in YieldDemo.SupplyIntegers())
  {
    Console.WriteLine(\"{0} is consumed by foreach iteration\         


        

Answer 1

Nope - far from it; I'll write a long-hand version for you... it is too grungy!

---

Note it also helps if you understand that the foreach is actually:

using(var iterator = YieldDemo.SupplyIntegers().GetEnumerator()) {
    int i;
    while(iterator.MoveNext()) {
        i = iterator.Current;
         Console.WriteLine("{0} is consumed by foreach iteration", i);
    }
}

---

using System;
using System.Collections;
using System.Collections.Generic;
static class Program
{
    static void Main()
    {

        foreach (int i in YieldDemo.SupplyIntegers())
        {
            Console.WriteLine("{0} is consumed by foreach iteration", i);
        }
    }
}

 class YieldDemo
  {

    public static IEnumerable SupplyIntegers()
     {
         return new YieldEnumerable();
       }
    class YieldEnumerable : IEnumerable
    {
        public IEnumerator GetEnumerator()
        {
            return new YieldIterator();
        }
        IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }
    }
    class YieldIterator : IEnumerator
    {
        private int state = 0;
        private int value;
        public int Current { get { return value; } }
        object IEnumerator.Current { get { return Current; } }
        void IEnumerator.Reset() { throw new NotSupportedException(); }
        void IDisposable.Dispose() { }
        public bool MoveNext()
        {
            switch (state)
            {
                case 0: value = 1; state = 1;  return true;
                case 1: value = 2; state = 2;  return true;
                case 2: value = 3; state = 3; return true;
                default: return false;
            }
        }
    }
}

As you can see, it builds a state machine in the iterator, with the state machine progressed by MoveNext. I've used the pattern with a state field, as you can see how this would work for more complex iterators.

Importantly:

• any variables in your iterator block become fields on the state machine
• if you have a finally block (including using), it goes in the Dispose()
• portions of code leading to a yield return become a case (roughly)
• yield break becomes a state = -1; return false; (or similar)

The way the C# compiler does this is very complicated, but it makes writing iterators a breeze.