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(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>
        public static DateTime LastDayOfMonth(this DateTime datetime)
        {
            return FirstDayOfMonth(datetime).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>
        public static DateTime FirstDayOfPreviousMonth(this DateTime datetime)
        {
            return FirstDayOfMonth(datetime).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>
        public static  DateTime LastDayOfPreviousMonth(this DateTime datetime)
        {
            return FirstDayOfMonth(datetime).AddDays(-1);
        }
 
        /// <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(-6);
        }
 
        /// <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 == 0 ? -7 : -dayOfWeek);
        }
    }
 
}