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);  
            }