寫好了，用壹個擴展方法實現，具體參考了國外壹個論壇上的答案，但是我作了壹些調整，感謝原作者Alexander！以下是截圖和核心代碼，完整的源碼在附件中，如有疑問歡迎追問。
註意：原方法寫得非常好，妳可以指定國定假日，但是在實例源碼中我並沒有用，妳自己註意看壹下。
核心代碼：
//?-----------------------------------------------------------------------//?<copyright?file="DateTimeExtention.cs"?Author="不識臺北路">
//?TODO:?Update?copyright?text.
//?</copyright>
//?-----------------------------------------------------------------------
namespace?WorkingDays
{
using?System;
///?<summary>
///?TODO:?Update?summary.
///?</summary>
public?static?class?DateTimeExtention
{
///?<summary>
///?Calculates?number?of?business?days,?taking?into?account:
///?-?weekends?(Saturdays?and?Sundays)
///?-?bank?holidays?in?the?middle?of?the?week
///?</summary>
///?<param?name="firstDay">First?day?in?the?time?interval</param>
///?<param?name="lastDay">Last?day?in?the?time?interval</param>
///?<param?name="bankHolidays">List?of?bank?holidays?excluding?weekends</param>
///?<returns>Number?of?business?days?during?the?'span'</returns>
public?static?int?BusinessDaysUntil(this?DateTime?firstDay,?DateTime?lastDay,?params?DateTime[]?bankHolidays)
{
firstDay?=?firstDay.Date;
lastDay?=?lastDay.Date;
if?(firstDay?>?lastDay)
throw?new?ArgumentException("Incorrect?last?day?"?+?lastDay);
TimeSpan?span?=?lastDay?-?firstDay;
int?businessDays?=?span.Days?+?1;
int?fullWeekCount?=?businessDays?/?7;
//?find?out?if?there?are?weekends?during?the?time?exceedng?the?full?weeks
if?(businessDays?>?fullWeekCount?*?7)
{
//?we?are?here?to?find?out?if?there?is?a?1-day?or?2-days?weekend
//?in?the?time?interval?remaining?after?subtracting?the?complete?weeks
int?firstDayOfWeek?=?firstDay.DayOfWeek?==?DayOfWeek.Sunday7?:?(int)firstDay.DayOfWeek;
int?lastDayOfWeek?=?lastDay.DayOfWeek?==?DayOfWeek.Sunday7?:?(int)lastDay.DayOfWeek;
if?(lastDayOfWeek?<?firstDayOfWeek)
lastDayOfWeek?+=?7;
if?(firstDayOfWeek?<=?6)
{
if?(lastDayOfWeek?>=?7)//?Both?Saturday?and?Sunday?are?in?the?remaining?time?interval
businessDays?-=?2;
else?if?(lastDayOfWeek?>=?6)//?Only?Saturday?is?in?the?remaining?time?interval
businessDays?-=?1;
}
else?if?(firstDayOfWeek?<=?7?&&?lastDayOfWeek?>=?7)//?Only?Sunday?is?in?the?remaining?time?interval
businessDays?-=?1;
}
//?subtract?the?weekends?during?the?full?weeks?in?the?interval
businessDays?-=?fullWeekCount?+?fullWeekCount;
//?subtract?the?number?of?bank?holidays?during?the?time?interval
foreach?(DateTime?bankHoliday?in?bankHolidays)
{
DateTime?bh?=?bankHoliday.Date;
if?(firstDay?<=?bh?&&?bh?<=?lastDay)
--businessDays;
}
return?businessDays;
}
}
}