C#求当前时间是一年中的第几周怎么算？

private int WeekOfYear(string date)
{
DateTime curDay = Convert.ToDateTime(date);

int firstdayofweek = Convert.ToInt32(Convert.ToDateTime(curDay.Year.ToString() + "- " + "1-1 ").DayOfWeek);

int days = curDay.DayOfYear;
int daysOutOneWeek = days - (7 - firstdayofweek);

if (daysOutOneWeek <= 0)
{
return 1;
}
else
{
int weeks = daysOutOneWeek / 7;
if (daysOutOneWeek % 7 != 0)
weeks++;

return weeks + 1;
}
}