下面的方法应该返回所需的DateTime。周日为true，周一为false:

private DateTime getStartOfWeek(bool useSunday)
{
    DateTime now = DateTime.Now;
    int dayOfWeek = (int)now.DayOfWeek;

    if(!useSunday)
        dayOfWeek--;

    if(dayOfWeek < 0)
    {// day of week is Sunday and we want to use Monday as the start of the week
    // Sunday is now the seventh day of the week
        dayOfWeek = 6;
    }

    return now.AddDays(-1 * (double)dayOfWeek);
}

public static System.DateTime getstartweek()
{
    System.DateTime dt = System.DateTime.Now;
    System.DayOfWeek dmon = System.DayOfWeek.Monday;
    int span = dt.DayOfWeek - dmon;
    dt = dt.AddDays(-span);
    return dt;
}

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

我和很多学校都有合作，所以正确使用周一作为一周的第一天在这里很重要。

这里很多最简洁的答案在周日都不起作用——我们经常在周日返回明天的日期，这不利于运行关于上周活动的报告。

这是我的解决方案，上周一周日，今天周一。

// Adding 7 so remainder is always positive; Otherwise % returns -1 on Sunday.
var daysToSubtract = (7 + (int)today.DayOfWeek - (int)DayOfWeek.Monday) % 7;

var monday = today
    .AddDays(-daysToSubtract)
    .Date;

记住为“today”使用一个方法参数，这样它就可以进行单元测试!!

使用扩展方法:

public static class DateTimeExtensions
{
    public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek)
    {
        int diff = (7 + (dt.DayOfWeek - startOfWeek)) % 7;
        return dt.AddDays(-1 * diff).Date;
    }
}

可以这样使用:

DateTime dt = DateTime.Now.StartOfWeek(DayOfWeek.Monday);
DateTime dt = DateTime.Now.StartOfWeek(DayOfWeek.Sunday);

把这些都放在一起，加上全球化并允许指定一周的第一天作为我们通话的一部分

public static DateTime StartOfWeek ( this DateTime dt, DayOfWeek? firstDayOfWeek )
{
    DayOfWeek fdow;

    if ( firstDayOfWeek.HasValue  )
    {
        fdow = firstDayOfWeek.Value;
    }
    else
    {
        System.Globalization.CultureInfo ci = System.Threading.Thread.CurrentThread.CurrentCulture;
        fdow = ci.DateTimeFormat.FirstDayOfWeek;
    }

    int diff = dt.DayOfWeek - fdow;

    if ( diff < 0 )
    {
        diff += 7;
    }

    return dt.AddDays( -1 * diff ).Date;

}