C# 获取某月的第一天和最后一天

/// <summary>        /// 取得某月的第一天        /// </summary>        /// <param name="datetime">要取得月份第一天的时间</param>        /// <returns></returns>        private DateTime FirstDayOfMonth(DateTime datetime)        {            return datetime.AddDays(1 - datetime.Day);        }        //// <summary>        /// 取得某月的最后一天        /// </summary>        /// <param name="datetime">要取得月份最后一天的时间</param>        /// <returns></returns>        private DateTime LastDayOfMonth(DateTime datetime)        {            return datetime.AddDays(1 - datetime.Day).AddMonths(1).AddDays(-1);        }        //// <summary>        /// 取得上个月第一天        /// </summary>        /// <param name="datetime">要取得上个月第一天的当前时间</param>        /// <returns></returns>        private DateTime FirstDayOfPreviousMonth(DateTime datetime)        {            return datetime.AddDays(1 - datetime.Day).AddMonths(-1);        }        //// <summary>        /// 取得上个月的最后一天         /// </summary>        /// <param name="datetime">要取得上个月最后一天的当前时间</param>        /// <returns></returns>        private DateTime LastDayOfPrdviousMonth(DateTime datetime)        {            return datetime.AddDays(1 - datetime.Day).AddDays(-1);        }

/// 获取指定日期，在为一年中为第几周

/// <param name="dt">指定时间</param>

/// <reutrn>返回第几周</reutrn>

private static int GetWeekOfYear(DateTime dt){ 

GregorianCalendar gc = new GregorianCalendar();

int weekOfYear = gc.GetWeekOfYear(dt,CalendarWeekRule.FirstDay, DayOfWeek.Monday);

return weekOfYear;}

}