C# 取得上月月头和月尾、上周的第一天和最后一天。

 using System;

 namespace ConsoleAppDemo
 {
     class Program
     {
         static void Main(string[] args)
         {
             DateTime now = DateTime.Now;
             Console.WriteLine($"First Day Of Month：{now.FirstDayOfMonth()}");
             Console.WriteLine($"Last Day Of Month：{now.LastDayOfMonth()}");
             Console.WriteLine($"First Day Of Previous Month：{now.FirstDayOfPreviousMonth()}");
             Console.WriteLine($"Last Day Of Previous Month：{now.LastDayOfPreviousMonth()}");
             Console.WriteLine($"First Day Of Previous Week：{now.FirstDayOfPreviousWeek()}");
             Console.WriteLine($"Last Day Of Previous Week：{now.LastDayOfPreviousWeek()}");
             Console.ReadKey();
         }
     }

     public static class DateTimeExtension
     {
         /// 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>
         public static DateTime FirstDayOfMonth(this DateTime datetime)
         {
             return datetime.AddDays( - 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>
         public static DateTime LastDayOfMonth(this DateTime datetime)
         {
             return FirstDayOfMonth(datetime).AddMonths().AddDays(-);
         }

         /// <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>
         public static DateTime FirstDayOfPreviousMonth(this DateTime datetime)
         {
             return FirstDayOfMonth(datetime).AddMonths(-);
         }

         /// <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>
         public static  DateTime LastDayOfPreviousMonth(this DateTime datetime)
         {
             return FirstDayOfMonth(datetime).AddDays(-);
         }

         /// <summary>
         /// 取得上周的第一天
         /// </summary>
         /// <param name="datetime">要取得上周第一天的当前时间</param>
         /// <returns></returns>
         public static DateTime FirstDayOfPreviousWeek(this DateTime datetime)
         {
             return LastDayOfPreviousWeek(datetime).AddDays(-);
         }

         /// <summary>
         /// 取得上周的最后一天
         /// </summary>
         /// <param name="datetime">要取得上周最后一天的当前时间</param>
         /// <returns></returns>
         public static DateTime LastDayOfPreviousWeek(this DateTime datetime)
         {
             int dayOfWeek = (int)datetime.DayOfWeek;
             return datetime.AddDays(dayOfWeek ==  ? - : -dayOfWeek);
         }
     }

 }