从Compile This' answer开始，使用以下方法获取一周中任何一天的日期:

public static DateTime GetDayOfWeek(DateTime dateTime, DayOfWeek dayOfWeek)
{
   var monday = dateTime.Date.AddDays((7 + (dateTime.DayOfWeek - DayOfWeek.Monday) % 7) * -1);

   var diff = dayOfWeek - DayOfWeek.Monday;

   if (diff == -1)
   {
      diff = 6;
   }

   return monday.AddDays(diff);
}

这将给你前一个星期天(我想):

DateTime t = DateTime.Now;
t -= new TimeSpan ((int) t.DayOfWeek, 0, 0, 0);

我是为周一做的，但对周日也有类似的逻辑。

public static DateTime GetStartOfWeekDate()
{
    // Get today's date
    DateTime today = DateTime.Today;
    // Get the value for today. DayOfWeek is an enum with 0 being Sunday, 1 Monday, etc
    var todayDayOfWeek = (int)today.DayOfWeek;

    var dateStartOfWeek = today;
    // If today is not Monday, then get the date for Monday
    if (todayDayOfWeek != 1)
    {
        // How many days to get back to Monday from today
        var daysToStartOfWeek = (todayDayOfWeek - 1);
        // Subtract from today's date the number of days to get to Monday
        dateStartOfWeek = today.AddDays(-daysToStartOfWeek);
    }

    return dateStartOfWeek;

}

使用Fluent DateTime:

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

    namespace DateTimeExample
    {
        using System;

        public static class DateTimeExtension
        {
            public static DateTime GetMonday(this DateTime time)
            {
                if (time.DayOfWeek != DayOfWeek.Monday)
                    return GetMonday(time.AddDays(-1)); //Recursive call

                return time;
            }
        }

        internal class Program
        {
            private static void Main()
            {
                Console.WriteLine(DateTime.Now.GetMonday());
                Console.ReadLine();
            }
        }
    } 

谢谢你的例子。我需要总是使用“CurrentCulture”一周的第一天，对于一个数组，我需要知道确切的天数。这里是我的第一个扩展:

public static class DateTimeExtensions
{
    //http://stackoverflow.com/questions/38039/how-can-i-get-the-datetime-for-the-start-of-the-week
    //http://stackoverflow.com/questions/1788508/calculate-date-with-monday-as-dayofweek1
    public static DateTime StartOfWeek(this DateTime dt)
    {
        //difference in days
        int diff = (int)dt.DayOfWeek - (int)CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek; //sunday=always0, monday=always1, etc.

        //As a result we need to have day 0,1,2,3,4,5,6 
        if (diff < 0)
        {
            diff += 7;
        }
        return dt.AddDays(-1 * diff).Date;
    }

    public static int DayNoOfWeek(this DateTime dt)
    {
        //difference in days
        int diff = (int)dt.DayOfWeek - (int)CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek; //sunday=always0, monday=always1, etc.

        //As a result we need to have day 0,1,2,3,4,5,6 
        if (diff < 0)
        {
            diff += 7;
        }
        return diff + 1; //Make it 1..7
    }
}