[C#]擷取某年指定周的開始日期和結束日期的通用方法

/// <summary>

/// 擷取一年中指定的一周的開始日期和結束日期。開始日期遵循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">确定首周的規則</param>

/// <param name="first">當此方法傳回時，則包含參數 year 和 weeks 指定的周的開始日期的 System.DateTime 值；如果失敗，則為 System.DateTime.MinValue。如果參數 year 或 weeks 超出有效範圍，則操作失敗。該參數未經初始化即被傳遞。</param>

/// <param name="last">當此方法傳回時，則包含參數 year 和 weeks 指定的周的結束日期的 System.DateTime 值；如果失敗，則為 System.DateTime.MinValue。如果參數 year 或 weeks 超出有效範圍，則操作失敗。該參數未經初始化即被傳遞。</param>

/// <returns>成功傳回 true，否則為 false。</returns>

public static bool GetDaysOfWeeks(int year, int weeks, CalendarWeekRule weekrule, out DateTime first, out DateTime last)

{

//初始化 out 參數

first = DateTime.MinValue;

last = DateTime.MinValue;

//不用解釋了吧...

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;

//取當年首日為基準...為什麼？容易得呗...

DateTime firstCurr = new DateTime(year, 1, 1);

//取下一年首日用于計算...

DateTime firstNext = new DateTime(year + 1, 1, 1);

//将當年首日星期幾轉換為數字...星期日特别處理...ISO 8601 标準...

int dayOfWeekFirst = (int)firstCurr.DayOfWeek;

if (dayOfWeekFirst == 0) dayOfWeekFirst = 7;

//得到未經驗證的周首日...

first = firstCurr.AddDays((weeks - 1) * 7 - dayOfWeekFirst + 1);

//周首日是上一年日期的情況...

if (first.Year < year)

{

switch (weekrule)

{

case CalendarWeekRule.FirstDay:

//不用解釋了吧...

first = firstCurr;

break;

case CalendarWeekRule.FirstFullWeek:

//順延一周...

first = first.AddDays(7);

break;

case CalendarWeekRule.FirstFourDayWeek:

//周首日距年首日不足4天則順延一周...

if (firstCurr.Subtract(first).Days > 3)

{

first = first.AddDays(7);

}

break;

default:

break;

}

}

//得到未經驗證的周末日...

last = first.AddDays(7).AddSeconds(-1);

switch (weekrule)

{

case CalendarWeekRule.FirstDay:

//周末日是下一年日期的情況...

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天則提前一周...

if (last.Year > year && firstNext.Subtract(first).Days < 4)

{

first = first.AddDays(-7);

last = last.AddDays(-7);

}

break;

default:

break;

}

return true;

}