using System;

using System.Collections.Generic;

​

public interface IGenerator<out T> : IEnumerator<T>, IEnumerable<T>

{

}

​

public class EnumerableWrapper<T> : IGenerator<T>

{

    private IEnumerable<T> wrapped;

    private IEnumerator<T> enumerator;

​

    public EnumerableWrapper(IEnumerable<T> wrapped) {

        this.wrapped = wrapped;

        this.enumerator = wrapped.GetEnumerator();

    }

​

    IEnumerator<T> GetEnumerator() { return this; }

​

    public T Current {

        get {

            return enumerator.Current;

        }

    }

    override public bool MoveNext() {

        return enumerator.MoveNext();

    }

    override public void Reset() {

        enumerator.Reset();

    }

    override public void Dispose() {

        enumerator.Dispose();

    }

}

​

public class C

{

    static IEnumerable<int> F()

    {

        Console.WriteLine("  F 0");

        yield return 0;

        Console.WriteLine("  F 1");

        yield return 1;

        Console.WriteLine("  F 2");

    }

​

    static IGenerator<int> FWrapped() {

        return new EnumerableWrapper<int>(F());

    }

​

    static int? GetNext(IEnumerable<int> e) {

        var en = e as IEnumerator<int>;

        return GetNext(en);

    }

​

    static int? GetNext(IEnumerator<int> e) {

        if (e.MoveNext()) {

            return e.Current;

        } else {

            return null;

        }

    }

​

    public static void Main()

    {

        Do1();

        Do2();

        Do3();

    }

​

    static void Do1() {

        var e = F().GetEnumerator();

        int count = 0;

        while (count++ < 10)

        {

            var i = GetNext(e);

            Console.WriteLine($"  Got {i}");

            if (i == null) { break; }

        }

        Console.WriteLine("  ForIn");

        //foreach (int i in F().GetEnumerator()) {

        //    Console.WriteLine(i);

        //}

    }

​

    static void Do2() {

        var e = F();

        int count = 0;

        while (count++ < 10)

        {

            var i = GetNext(e);

            Console.WriteLine($"  Got {i}");

            if (i == null) { break; }

        }

        Console.WriteLine("  ForIn");

        foreach (int i in F()) {

            Console.WriteLine(i);

        }

    }

​

    static void Do3() {

        var e = FWrapped();

        int count = 0;

        while (count++ < 10)

        {

            var i = GetNext(e as IEnumerator<int>);

            Console.WriteLine($"  Got {i}");

            if (i == null) { break; }

        }

        Console.WriteLine("  ForIn");

        foreach (int i in FWrapped()) {

            Console.WriteLine(i);

        }

    }

}

​