C#擷取周一、周日的日期 函數類

#region 得到一周的周一和周日的日期
        /// <summary> 
        /// 計算本周的周一日期 
        /// </summary> 
        /// <returns></returns> 
        public static DateTime GetMondayDate()
        {
            return GetMondayDate(DateTime.Now);
        }
        /// <summary> 
        /// 計算本周周日的日期 
        /// </summary> 
        /// <returns></returns> 
        public static DateTime GetSundayDate()
        {
            return GetSundayDate(DateTime.Now);
        }
        /// <summary> 
        /// 計算某日起始日期（禮拜一的日期） 
        /// </summary> 
        /// <param name="someDate">該周中任意一天</param> 
        /// <returns>返回禮拜一日期，後面的具體時、分、秒和傳入值相等</returns> 
        public static DateTime GetMondayDate(DateTime someDate)
        {
            int i = someDate.DayOfWeek - DayOfWeek.Monday;
            if (i == -1) i = 6;// i值 > = 0 ，因為枚舉原因，Sunday排在最前，此時Sunday-Monday=-1，必須+7=6。 
            TimeSpan ts = new TimeSpan(i, 0, 0, 0);
            return someDate.Subtract(ts);
        }
        /// <summary> 
        /// 計算某日結束日期（禮拜日的日期） 
        /// </summary> 
        /// <param name="someDate">該周中任意一天</param> 
        /// <returns>返回禮拜日日期，後面的具體時、分、秒和傳入值相等</returns> 
        public static DateTime GetSundayDate(DateTime someDate)
        {
            int i = someDate.DayOfWeek - DayOfWeek.Sunday;
            if (i != 0) i = 7 - i;// 因為枚舉原因，Sunday排在最前，相減間隔要被7減。 
            TimeSpan ts = new TimeSpan(i, 0, 0, 0);
            return someDate.Add(ts);
        }
        #endregion