c# - "semi-monthly"加多少天

Question

我有一个名为 PaymentFrequency 的枚举类型，它的值表示每年支付了多少...所以我有

public enum PaymentFrequency
{
    None             = 0,
    Annually         = 1,
    SemiAnnually     = 2,
    EveryFourthMonth = 3,
    Quarterly        = 4,
    BiMonthly        = 6,
    Monthly          = 12,
    EveryFourthWeek  = 13,
    SemiMonthly      = 24,
    BiWeekly         = 26,
    Weekly           = 52
}

基于 NumberOfPayments、PaymentFrequency 和 FirstPaymentDate(DateTimeOffset 类型)我想计算 LastPaymentDate。但是我无法确定在 SemiMonthly 的情况下要添加多少时间单位(天、月)...

    switch (paymentFrequency)
    {
        // add years...
        case PaymentFrequency.Annually:
            LastPaymentDate = FirstPaymentDate.AddYears(NumberOfPayments - 1); 
            break;
        // add months...
        case PaymentFrequency.SemiAnnually:
            LastPaymentDate = FirstPaymentDate.AddMonths((NumberOfPayments - 1) * 6); // 6 months
            break;
        case PaymentFrequency.EveryFourthMonth:
            LastPaymentDate = FirstPaymentDate.AddMonths((NumberOfPayments - 1) * 4); // 4 months
            break;
        case PaymentFrequency.Quarterly:
            LastPaymentDate = FirstPaymentDate.AddMonths((NumberOfPayments - 1) * 3); // 3 months
            break;
        case PaymentFrequency.BiMonthly:
            LastPaymentDate = FirstPaymentDate.AddMonths((NumberOfPayments - 1) * 2); // 2 months
            break;
        case PaymentFrequency.Monthly:
            LastPaymentDate = FirstPaymentDate.AddMonths(NumberOfPayments - 1);
            break;
        // add days...
        case PaymentFrequency.EveryFourthWeek:
            LastPaymentDate = FirstPaymentDate.AddDays((NumberOfPayments - 1) * 4 * 7); // 4 weeks (1 week = 7 days)
            break;
        case PaymentFrequency.SemiMonthly:
            // NOTE: how many days in semi month? AddMonths (0.5) does not work :)
            LastPaymentDate = FirstPaymentDate.AddMonths((NumberOfPayments - 1) * 0.5); // 2 weeks (1 week = 7 days)
            break;
        case PaymentFrequency.BiWeekly:
            LastPaymentDate = FirstPaymentDate.AddDays((NumberOfPayments - 1) * 2 * 7); // 2 weeks (1 week = 7 days)
            break;
        case PaymentFrequency.Weekly:
            LastPaymentDate = FirstPaymentDate.AddDays((NumberOfPayments - 1) * 7); // 1 week (1 week = 7 days)
            break;
        case PaymentFrequency.None:
        default:
            throw new ArgumentException("Payment frequency is not initialized to valid value!", "paymentFrequency");
    }

那么，在使用 SemiMonthly 时，我应该使用多少天/月？如果不知道中间每个月的确切天数，这甚至有可能吗？或者这真的很简单，我刚刚喝完咖啡因，只见树木不见森林 :)

Answer 1

对于半月，如果您的第一笔付款也始终是当月的第一笔付款(即，从 1 日到 13 日的任何时间，如评论中所讨论的那样，从 13 日之后开始是有问题的)，您可以这样做如下:

 // assuming first payment will be 1st of month, add month for every 2 payments
 // num payments / 2 (int division, remainder is chucked)
 // then add 15 days if this is even payment of the month
 LastPaymentDate = FirstPaymentDate.AddMonths((NumberOfPayments - 1) / 2)
     .AddDays((NumberOfPayments % 2) == 0 ? 15 : 0);

因此对于第一次付款，这将增加 0 个月和 0 天，因此是第一次付款日期。对于第 2 次付款，这将增加 0 个月(整数除法，剩余部分被剔除)和 16 日的 15 天。对于第 3 次付款，这将增加 1 个月 (1/3) 和下个月 1 日的 0 天，依此类推。

这是假设 FirstPaymentDate 将在某个给定月份的 1 号。如果您想将 16 日作为开始日期等，您可能会从这里看到该去哪里。

有道理吗？

所以为了说明，如果我们有:

DateTime LastPaymentDate, FirstPaymentDate = new DateTime(2011, 12, 5);

for(int numOfPayments=1; numOfPayments<=24; numOfPayments++)
{
    LastPaymentDate = FirstPaymentDate.AddMonths((numOfPayments - 1) / 2)
        .AddDays((numOfPayments % 2) == 0 ? 15 : 0);

    Console.WriteLine(LastPaymentDate);
}

这个循环会给我们:

12/5/2011 12:00:00 AM
12/20/2011 12:00:00 AM
1/5/2012 12:00:00 AM
// etc...
10/20/2012 12:00:00 AM
11/5/2012 12:00:00 AM
11/20/2012 12:00:00 AM