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c#enumerationyieldyield-return

The wonders of the yield keyword

Ok, as I was poking around with building a custom enumerator, I had noticed this behavior that concerns the yield

Say you have something like this:

  public class EnumeratorExample 
  {

        public static IEnumerable<int> GetSource(int startPoint) 
        {
                int[] values = new int[]{1,2,3,4,5,6,7};
                Contract.Invariant(startPoint < values.Length);
                bool keepSearching = true;
                int index = startPoint;

                while(keepSearching) 
                {
                      yield return values[index];
                      //The mind reels here
                      index ++ 
                      keepSearching = index < values.Length;
                }
        }

  }

What makes it possible underneath the compiler's hood to execute the index ++ and the rest of the code in the while loop after you technically do a return from the function?

Solution

  • The compiler rewrites the code into a state machine. The single method you wrote is split up into different parts. Each time you call MoveNext (either implicity or explicitly) the state is advanced and the correct block of code is executed.

    Suggested reading if you want to know more details: