using System;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("本周第一天：{0}", GetWeekFirstDate(DateTime.Now));

            Console.Read();
        }

        /// <summary>  
        /// 獲取本周第一天（以星期一為第一天）
        /// </summary>  
        /// <param name="dateTime">當前時間 DateTime.Now or DateTime.UtcNow</param>  
        /// <returns>后面的具體時、分、秒和傳入值相等</returns>  
        public static DateTime GetWeekFirstDate(DateTime dateTime)
        {
            int weeknow = Convert.ToInt32(dateTime.DayOfWeek);

            // 因為是以星期一為第一天，所以要判斷weeknow等于0時，要向前推6天。
            weeknow = (weeknow == 0 ? (7 - 1) : (weeknow - 1));
            int daydiff = (-1) * weeknow;

            // 本周第一天
            return Convert.ToDateTime(dateTime.AddDays(daydiff));
        }
    }
}