using System;

using System.Collections;

​

public static class Ext {

    public static IEnumerator GetRecursiveEnumerator(this IEnumerable i) {

        foreach (object v in i) {

            if (v is IEnumerable) {

                var _v = v as IEnumerable;

                var _n = _v.GetRecursiveEnumerator();

                while (_n.MoveNext()) {

                    yield return _n.Current;

                }

            }

            else {

                yield return v;

            }

        }

        yield break;

    }

}

​

public class Program

{

    public static IEnumerable f0() {

        yield return null;

        Console.WriteLine("f0 1");

        yield return null;

        Console.WriteLine("f0 2");

        yield return null;

    }

    public static IEnumerable f1() {

        yield return null;

        Console.WriteLine("f1 1");

        yield return f0();

        Console.WriteLine("f1 2");

        yield return f0();

    }

    public static IEnumerable f2() {

        Console.WriteLine("f2 0");

        yield return f1();

        Console.WriteLine("f2 1");

        yield return null;

    }

    public static void Main()

    {

        var f = f2().GetRecursiveEnumerator();

        while (f.MoveNext()) {

        }

    }

}