Question

When i have a code block

static void Main()
{

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


 class YieldDemo
  {
    public static IEnumerable<int> SupplyIntegers()
     {
         yield return 1;
         yield return 2;
          yield return 3;
       }
   }

can i interpret the principle behind yield return as

1. Main() calls the SupplyIntegers()
2. |1| |2| |3| are stored in contiguous memory block.Pointer of "IEnumerator" Moves to |1|
3. Control returns from SupplyInteger() to Main().
4. Main() prints the value
5. Pointer Moves to |2|, and so on.

Clarifications :

(1) Normally we will have one valid return statement is allowed inside a function.How does
C# treats when multiple yield return ,yield return,… statements are present?

(2) Once the return is encountered there is no way for control again coming back to
SupplyIntegers(), in case it is allowed won’t the Yield again starts from 1 ? I mean yield return 1?

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<int> SupplyIntegers()
     {
         return new YieldEnumerable();
       }
    class YieldEnumerable : IEnumerable<int>
    {
        public IEnumerator<int> GetEnumerator()
        {
            return new YieldIterator();
        }
        IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }
    }
    class YieldIterator : IEnumerator<int>
    {
        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.