C# 中根据datetime的值来计算属于本年的第几周,类似delphi中的weekoftheyear功能

         /// <summary>
         /// 获得今天是今年的第几周
         /// </summary>
         /// <param name="year"></param>
         /// <returns></returns>
         public string GetWeekOfYear(DateTime today)
         {
            int days = today.DayOfYear;
            int remainder = days % ;
            if (remainder == )
                return Convert.ToString(days / );
             else
                return Convert.ToString(days /  + );
         }

-------------上面的代码没有考虑到当年的1月1号，并非是周一，所以算出来的会有误差

下面是参考博客园唔愛吃蘋果的代码。

         public string GetWeekOfYear(DateTime today)
         {
            int weeknow = Convert.ToInt32(today.DayOfWeek);//today是周几
            int daydiff = (-) * (weeknow + );//今日与上周末的天数差
            int days = today.AddDays(daydiff).DayOfYear;//上周末是本年第几天
            int weeks = days / ;
            if (days %  != )
            {
                weeks++;
            }
            //此时，weeks为上周是本年的第几周
            return Convert.ToString(weeks + );
         }

他这边就巧妙的用上周末的来计算之前的周数

可是也存在比如本年的1月1号是周三，那么上周的周末是去年的，导致计算出错。

再加入判断，修改后代码如下：

         /// <summary>
         /// 获得今天是今年的第几周
         /// </summary>
         /// <param name="today"></param>
         /// <returns></returns>
         public string GetWeekOfYear(DateTime today)
         {
             int NewYear = Convert.ToInt32(today.Year); //今年的年份
             int weeknow = Convert.ToInt32(today.DayOfWeek);//today是周几
             int daydiff = (- * weeknow);//今日与上周末的天数差
             int days = today.AddDays(daydiff).DayOfYear;//上周末是本年第几天
             int OldYear = Convert.ToInt32(today.AddDays(daydiff).Year); //上周末的年份
             if (NewYear == OldYear)
             {
                int weeks = days / ;
                if (days %  != )
                {
                    weeks++;
                }
                //此时，weeks为上周是本年的第几周
                return Convert.ToString(weeks + );
             }
             else
                return "";
         }