[C#]获取某年指定周的开始日期和结束日期的通用方法

/// <summary>
/// 获取一年中指定的一周的开始日期和结束日期。开始日期遵循ISO 8601即星期一。
/// </summary>
/// <remarks>Write by vrhero</remarks>
/// <param name="year">年（1 到 9999）</param>
/// <param name="weeks">周（1 到 53）</param>
/// <param name="weekrule">确定首周的规则</param>
/// <param name="first">当此方法返回时，则包含参数 year 和 weeks 指定的周的开始日期的 System.DateTime 值；如果失败，则为 System.DateTime.MinValue。如果参数 year 或 weeks 超出有效范围，则操作失败。该参数未经初始化即被传递。</param>
/// <param name="last">当此方法返回时，则包含参数 year 和 weeks 指定的周的结束日期的 System.DateTime 值；如果失败，则为 System.DateTime.MinValue。如果参数 year 或 weeks 超出有效范围，则操作失败。该参数未经初始化即被传递。</param>
/// <returns>成功返回 true，否则为 false。</returns>
public static bool GetDaysOfWeeks(int year, int weeks, CalendarWeekRule weekrule, out DateTime first, out DateTime last)
{
//初始化 out 参数
first = DateTime.MinValue;
last = DateTime.MinValue;

//不用解释了吧...
if (year < 1 | year > 9999)
return false;

//一年最多53周地球人都知道...
if (weeks < 1 | weeks > 53)
return false;

//取当年首日为基准...为什么？容易得呗...
DateTime firstCurr = new DateTime(year, 1, 1);
//取下一年首日用于计算...
DateTime firstNext = new DateTime(year + 1, 1, 1);

//将当年首日星期几转换为数字...星期日特别处理...ISO 8601 标准...
int dayOfWeekFirst = (int)firstCurr.DayOfWeek;
if (dayOfWeekFirst == 0) dayOfWeekFirst = 7;

//得到未经验证的周首日...
first = firstCurr.AddDays((weeks - 1) * 7 - dayOfWeekFirst + 1);

//周首日是上一年日期的情况...
if (first.Year < year)
{
switch (weekrule)
{
case CalendarWeekRule.FirstDay:
//不用解释了吧...
first = firstCurr;
break;
case CalendarWeekRule.FirstFullWeek:
//顺延一周...
first = first.AddDays(7);
break;
case CalendarWeekRule.FirstFourDayWeek:
//周首日距年首日不足4天则顺延一周...
if (firstCurr.Subtract(first).Days > 3)
{
first = first.AddDays(7);
}
break;
default:
break;
}
}
//得到未经验证的周末日...
last = first.AddDays(7).AddSeconds(-1);
switch (weekrule)
{
case CalendarWeekRule.FirstDay:
//周末日是下一年日期的情况...
if (last.Year > year)
last = firstNext.AddSeconds(-1);
else if (last.DayOfWeek != DayOfWeek.Monday)
last = first.AddDays(7 - (int)first.DayOfWeek).AddSeconds(-1);
break;
case CalendarWeekRule.FirstFourDayWeek:
//周末日距下一年首日不足4天则提前一周...
if (last.Year > year && firstNext.Subtract(first).Days < 4)
{
first = first.AddDays(-7);
last = last.AddDays(-7);
}
break;
default:
break;
}
return true;
}