c# – 语法问题使用yield return的IEnumerable方法

static IEnumerable<DateTime> GetMonths(DateTime from,DateTime to)
{
    // if logs is not uptodate
    TimeSpan logsMissingTimespan = to - from;

    if (logsMissingTimespan != new TimeSpan(0))
    {
        return GetMonthsBetweenTwoDates(from,to);
    }

    return null; // Why this line ?
}

private static IEnumerable<DateTime> GetMonthsBetweenTwoDates(DateTime from,DateTime to)
{

    DateTime date = from;
    DateTime lastDate = DateTime.MaxValue;

    while (date < to)
    {
        if (lastDate.Month != date.Month)
        {
            lastDate = date;
            yield return lastDate;
        }
        date = date.AddDays(1);
    }
}

它工作正常,但我想我可以写一些像这样清洁的东西：

static IEnumerable<DateTime> GetMonths(DateTime from,DateTime to)
{
    TimeSpan logsMissingTimespan = to - from;

    if (logsMissingTimespan == new TimeSpan(0))
    {
        yield break;
    }

    return GetMonthsBetweenTwoDates(from,to);
}

但是我有一条错误消息：

Cannot return a value from an iterator. Use the yield return statement to return a value,or yield break to end the iteration.

为什么我应该返回null并且语法正确？

编辑：

所以,正确的方法是使用Enumerable.Empty：

static IEnumerable<DateTime> GetMonths(DateTime from,to);
    }

    return Enumerable.Empty<DateTime>();
}