6.2.3 迭代器的执行流程（yield break 语句）

static IEnumerable<int> CountWithTimeLimit(DateTime limit)
{
    for (int i = 1; i <= 100; i++)
    {
        if (DateTime.Now >= limit)
            yield break;
        yield return i;
    }
}

static void Main(string[] args)
{
    DateTime stop = DateTime.Now.AddSeconds(2);
    foreach (int i in CountWithTimeLimit(stop))
    {
        Console.WriteLine("Received{0}", i);
        Thread.Sleep(300);
    }
}