C# 根据年月获得此月第一天和最后一天，并计算工作日

string str = "2015年3月";
int firstIndex = str.IndexOf("年", 0);
int secondIndex = str.IndexOf("月", firstIndex + 1);
string month = str.Substring(firstIndex + 1, secondIndex - firstIndex - 1);
string year = str.Substring(0, 4);
DateTime dt = DateTime.Now;
string[] time = dt.ToString().Split(new char[]{‘/‘});
string date = year + "/" + month + "/" + time[2];
DateTime first = Convert.ToDateTime(date).AddDays(1 - Convert.ToDateTime(date).Day);
DateTime last=Convert.ToDateTime(date).AddDays(1 - Convert.ToDateTime(date).Day).AddMonths(1).AddDays(-1);
//The first day of the month
Label1.Text = first.ToString();
//The last day of the month
Label2.Text = last.ToString();
//businessDays of the month(except on the weekend)
Label3.Text = getint(first, last).ToString();

public int getint(DateTime firstDay, DateTime lastDay)
{
    firstDay = firstDay.Date;
    lastDay = lastDay.Date;
    if (firstDay > lastDay)
        throw new ArgumentException("Incorrect last day " + lastDay);
    TimeSpan span = lastDay - firstDay;
    int businessDays = span.Days + 1;
    int fullWeekCount = businessDays / 7;
    // find out if there are weekends during the time exceedng the full weeks
    if (businessDays > fullWeekCount * 7)
    {
        // we are here to find out if there is a 1-day or 2-days weekend
        // in the time interval remaining after subtracting the complete weeks
        int firstDayOfWeek = firstDay.DayOfWeek == DayOfWeek.Sunday ? 7 : (int)firstDay.DayOfWeek;
        int lastDayOfWeek = lastDay.DayOfWeek == DayOfWeek.Sunday ? 7 : (int)lastDay.DayOfWeek;

        if (lastDayOfWeek < firstDayOfWeek)
            lastDayOfWeek += 7;
        if (firstDayOfWeek <= 6)
        {
            if (lastDayOfWeek >= 7)// Both Saturday and Sunday are in the remaining time interval
                businessDays -= 2;
            else if (lastDayOfWeek >= 6)// Only Saturday is in the remaining time interval
                businessDays -= 1;
        }
        else if (firstDayOfWeek <= 7 && lastDayOfWeek >= 7)// Only Sunday is in the remaining time interval
            businessDays -= 1;
    }
    // subtract the weekends during the full weeks in the interval
    businessDays -= fullWeekCount + fullWeekCount;
    return businessDays;
}