6.2.2 观察迭代器的工作流程

static readonly string Padding = new string(' ', 30);

static IEnumerable<int> CreateEnumerable()
{
    Console.WriteLine("{0}start of CreateEnumerable()", Padding);
    for (int i = 0; i < 3; i++)
    {
        Console.WriteLine("{0}about to yield {1}", Padding, i);
        yield return i;
        Console.WriteLine("{0}after yield", Padding);
    }

    Console.WriteLine("{0}yielding final value", Padding);
    yield return -1;
    Console.WriteLine("{0}end of CreateEnumerable()", Padding);
}

static void Main(string[] args)
{
    IEnumerable<int> iterable = CreateEnumerable();
    IEnumerator<int> iterator = iterable.GetEnumerator();
    Console.WriteLine("starting to iterate");
    while (true)
    {
        Console.WriteLine("calling MoveNext()....");
        bool result = iterator.MoveNext();
        Console.WriteLine("...MoveNext result={0}", result);
        if (!result)
        {
            break;
        }
        Console.WriteLine("fetching current....");
        Console.WriteLine("...Current result={0}", iterator.Current);
    }
}