使用Fluent DateTime:

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

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;
    }

我是这样做的:

DateTime.Now.Date.AddDays(-(DateTime.Now.Date.DayOfWeek == 0 ? 7 : (int)DateTime.Now.Date.DayOfWeek) + 1)

这段代码所做的就是从给定的日期时间中减去天数。

如果星期几是0(星期日)，那么减去7，否则减去星期几。

然后在前一行的结果上加上1天，这将给出该日期的星期一。

这样你就可以在最后摆弄数字(1)来得到你想要的那一天。

private static DateTime GetDay(DateTime date, int daysAmount = 1)
{
    return date.Date.AddDays(-(date.Date.DayOfWeek == 0 ? 7 : (int)date.Date.DayOfWeek) + daysAmount);
}

如果你真的想使用DayOfWeek枚举，那么可以使用这样的东西…虽然我个人更喜欢上面的那个，因为我可以增加或减少任何数量的天数。

private static DateTime GetDayOfWeek(DateTime date, DayOfWeek dayOfWeek = DayOfWeek.Monday)
{
    return date.Date.AddDays(-(date.Date.DayOfWeek == 0 ? 7 : (int)date.Date.DayOfWeek) + (dayOfWeek == 0 ? 7 : (int)dayOfWeek));
}

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();
        }
    }
}

如果您需要周六、周日或一周中的任何一天，但不超过当前一周(周六-日)，我用这段代码为您提供了支持。

public static DateTime GetDateInCurrentWeek(this DateTime date, DayOfWeek day)
{
    var temp = date;
    var limit = (int)date.DayOfWeek;
    var returnDate = DateTime.MinValue;

    if (date.DayOfWeek == day) 
        return date;

    for (int i = limit; i < 6; i++)
    {
        temp = temp.AddDays(1);

        if (day == temp.DayOfWeek)
        {
            returnDate = temp;
            break;
        }
    }
    if (returnDate == DateTime.MinValue)
    {
        for (int i = limit; i > -1; i++)
        {
            date = date.AddDays(-1);

            if (day == date.DayOfWeek)
            {
                returnDate = date;
                break;
            }
        }
    }
    return returnDate;
}

我尝试了几次，但我没有解决从星期一开始的一周的问题，导致我的下一个星期一是星期天。所以我对它做了一些修改，让它与下面的代码一起工作:

int delta = DayOfWeek.Monday - DateTime.Now.DayOfWeek;
DateTime monday = DateTime.Now.AddDays(delta == 1 ? -6 : delta);
return monday;