c# 自然周

　　　//根據年份計算該年有多少周

public static int GetWeekCountOfYear(int strYear)

        {

            DateTime firstDay = DateTime.Parse(strYear.ToString() + "-01-01");

int weekOfFirstDay = Convert.ToInt32(firstDay.DayOfWeek);//得到該年的第一天是星期幾

int countWeek;//該年有多少周

//

//一年最少有53周，最多有54周

//如果是平年（365），52×7=364，多餘1天，故共有53周

//如果是閏年（366），52×7=364，多餘2天：

////如果多餘的兩天分別是年初和年末，則有54周，其他情況均為53周

//

if (DateTime.IsLeapYear(strYear) && weekOfFirstDay == 7)

            {//如果該年是閏年且第一天為星期日

                countWeek = 54;

            }

else

            {

                countWeek = 53;

            }

return countWeek;

        }



//計算某天在某年的第幾周內,且返回這周的起始日期

public int GetWeekOfCurrDate(DateTime dt)

        {

int week = 0;//返回的周次

//擷取該年有多少周

int countWeek = 0;

            countWeek = GetWeekCountOfYear(dt.Year);



//擷取該年有多少天

int countDay = 0;

if (!DateTime.IsLeapYear(dt.Year))

            {

//平年

                countDay = 365;

            }

else

            {

//閏年

                countDay = 366;

            }



//擷取該年第一天是星期幾

int firstDayOfWeek = (int)DateTime.Parse(dt.Year.ToString() + "-01-01").DayOfWeek;



//擷取該年最後一天是星期幾

int lastDayOfWeek = (int)DateTime.Parse(dt.Year.ToString() + "-12-31").DayOfWeek;



//擷取第一周有多少天

int countDayOfFirstWeek = 7 - firstDayOfWeek + 1;



//擷取最後一周有多少天

int countDayOfLastWeek = lastDayOfWeek;



//除去第一周和最後一周，該年還剩多少天

int countDayRemain = countDay - countDayOfFirstWeek - countDayOfLastWeek;



//除去第一周和最後一周，該年還剩多少周

int countWeekRemain = countDayRemain % 7;



if (dt.DayOfYear <= countDayOfFirstWeek)

            {

//所求的日期在第一周內

                week = 1;

            }

else

            {

if (dt.DayOfYear >= countDay - countDayOfLastWeek)

                {

//所求的日期在最後一周內

                    week = countWeek - 2;

                }

else

                {

//判斷除去第一周，所求日期是否能被7整除

if ((dt.DayOfYear - countDayOfFirstWeek) / 7 == 0)

                    {

                        week = (dt.DayOfYear - countDayOfFirstWeek) / 7 + 1;

                    }

else

                    {

                        week = (dt.DayOfYear - countDayOfFirstWeek) / 7 + 2;

                    }

                }

            }

return week;

        }