【转载】对C#DateTime的一些扩展,计算周内第一天，最后一天

    /// <summary>
    /// DateTime的一些扩展
    /// </summary>
    public class DateTime2
    {
        /// <summary>
        /// 得到本周第一天(以星期天为第一天)
        /// </summary>
        /// <param name="datetime"></param>
        /// <returns></returns>
        public DateTime GetWeekFirstDaySun(DateTime datetime)
        {
            //星期天为第一天
            int weeknow = Convert.ToInt32(datetime.DayOfWeek);
            int daydiff = (-1) * weeknow;

            //本周第一天
            string FirstDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
            return Convert.ToDateTime(FirstDay);
        }

        /// <summary>
        /// 得到本周第一天(以星期一为第一天)
        /// </summary>
        /// <param name="datetime"></param>
        /// <returns></returns>
        public DateTime GetWeekFirstDayMon(DateTime datetime)
        {
            //星期一为第一天
            int weeknow = Convert.ToInt32(datetime.DayOfWeek);

            //因为是以星期一为第一天，所以要判断weeknow等于0时，要向前推6天。
            weeknow = (weeknow == 0 ? (7 - 1) : (weeknow - 1));
            int daydiff = (-1) * weeknow;

            //本周第一天
            string FirstDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
            return Convert.ToDateTime(FirstDay);
        }

        /// <summary>
        /// 得到本周最后一天(以星期六为最后一天)
        /// </summary>
        /// <param name="datetime"></param>
        /// <returns></returns>
        public DateTime GetWeekLastDaySat(DateTime datetime)
        {
            //星期六为最后一天
            int weeknow = Convert.ToInt32(datetime.DayOfWeek);
            int daydiff = (7 - weeknow) - 1;

            //本周最后一天
            string LastDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
            return Convert.ToDateTime(LastDay);
        }

        /// <summary>
        /// 得到本周最后一天(以星期天为最后一天)
        /// </summary>
        /// <param name="datetime"></param>
        /// <returns></returns>
        public DateTime GetWeekLastDaySun(DateTime datetime)
        {
            //星期天为最后一天
            int weeknow = Convert.ToInt32(datetime.DayOfWeek);
            weeknow = (weeknow == 0 ? 7 : weeknow);
            int daydiff = (7 - weeknow);

            //本周最后一天
            string LastDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
            return Convert.ToDateTime(LastDay);
        }
    }

原文链接：http://blog.csdn.net/joyhen/article/details/9179763