d = DateTime.Now;
            int dayofweek =(int) d.DayOfWeek;
            if (dayofweek != 0)
            {
                d = d.AddDays(1 - dayofweek);
            }
            else { d = d.AddDays(-6); }

c#中的模数对-1 mod 7(它应该是6，但c#返回-1)效果很差 所以…一个“一行程序”的解决方案是这样的:

private static DateTime GetFirstDayOfWeek(DateTime date)
{
    return date.AddDays(
        -(((int)date.DayOfWeek - 1) -
          (int)Math.Floor((double)((int)date.DayOfWeek - 1) / 7) * 7));
}

使用Fluent DateTime:

var monday = DateTime.Now.Previous(DayOfWeek.Monday);
var sunday = DateTime.Now.Previous(DayOfWeek.Sunday);

我们喜欢简单的语句:获取当前文化中每周的第一天与当前日期之间的差值，然后从当前日期减去天数:

var weekStartDate = DateTime.Now.AddDays(-((int)now.DayOfWeek - (int)DateTimeFormatInfo.CurrentInfo.FirstDayOfWeek));

虽然很丑，但至少还能说出正确的日期

由系统设置一周开始:

    public static DateTime FirstDateInWeek(this DateTime dt)
    {
        while (dt.DayOfWeek != System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat.FirstDayOfWeek)
            dt = dt.AddDays(-1);
        return dt;
    }

没有:

    public static DateTime FirstDateInWeek(this DateTime dt, DayOfWeek weekStartDay)
    {
        while (dt.DayOfWeek != weekStartDay)
            dt = dt.AddDays(-1);
        return dt;
    }

using System;
using System.Globalization;

namespace MySpace
{
    public static class DateTimeExtention
    {
        // ToDo: Need to provide culturaly neutral versions.

        public static DateTime GetStartOfWeek(this DateTime dt)
        {
            DateTime ndt = dt.Subtract(TimeSpan.FromDays((int)dt.DayOfWeek));
            return new DateTime(ndt.Year, ndt.Month, ndt.Day, 0, 0, 0, 0);
        }

        public static DateTime GetEndOfWeek(this DateTime dt)
        {
            DateTime ndt = dt.GetStartOfWeek().AddDays(6);
            return new DateTime(ndt.Year, ndt.Month, ndt.Day, 23, 59, 59, 999);
        }

        public static DateTime GetStartOfWeek(this DateTime dt, int year, int week)
        {
            DateTime dayInWeek = new DateTime(year, 1, 1).AddDays((week - 1) * 7);
            return dayInWeek.GetStartOfWeek();
        }

        public static DateTime GetEndOfWeek(this DateTime dt, int year, int week)
        {
            DateTime dayInWeek = new DateTime(year, 1, 1).AddDays((week - 1) * 7);
            return dayInWeek.GetEndOfWeek();
        }
    }
}