C#取兩個時間的時間差并去除周末

C#取兩個時間的時間差并去除周末，取時間差的代碼很多，但是能夠只取工作日的時間差的代碼很少

protected void Page_Load(object sender, EventArgs e)
    {

DateTime start = Convert.ToDateTime("2012-12-10");
        DateTime end= Convert.ToDateTime("2012-12-18");
        TimeSpan span = end - start;
        //int totleDay=span.Days;
        //DateTime spanNu = DateTime.Now.Subtract(span);
        int AllDays=Convert.ToInt32(span.TotalDays)+1;//差距的所有天數
        int totleWeek = AllDays / 7;//差別多少周
        int yuDay = AllDays % 7; //除了整個星期的天數
        int lastDay = 0;
        if (yuDay == 0) //正好整個周
        {
            lastDay = AllDays - (totleWeek * 2);
        }
        else
        {

      int weekDay = 0;
      int endWeekDay = 0;  //多余的天數有幾天是周六或者周日
      switch (start.DayOfWeek)
      {
          case DayOfWeek.Monday:
              weekDay = 1;
              break;
          case DayOfWeek.Tuesday:
              weekDay = 2;
              break;
          case DayOfWeek.Wednesday:
              weekDay = 3;
              break;
          case DayOfWeek.Thursday:
              weekDay = 4;
              break;
          case DayOfWeek.Friday:
              weekDay = 5;
              break;
          case DayOfWeek.Saturday:
              weekDay = 6;
              break;
          case DayOfWeek.Sunday:
              weekDay = 7;
              break;
      }
      if ((weekDay == 6 && yuDay >= 2) || (weekDay == 7 && yuDay >= 1) || (weekDay == 5 && yuDay >= 3) || (weekDay == 4 && yuDay >= 4) || (weekDay == 3 && yuDay >= 5) || (weekDay == 2 && yuDay >= 6) || (weekDay == 1 && yuDay >=7))
      {
          endWeekDay =2;
      }
      if ((weekDay == 6 && yuDay < 1) || (weekDay == 7 && yuDay <5) || (weekDay == 5 && yuDay < 2) || (weekDay == 4 && yuDay < 3) || (weekDay == 3 && yuDay < 4) || (weekDay == 2 && yuDay < 5) || (weekDay == 1 && yuDay < 6))          {
          endWeekDay = 1;
      }
      lastDay = AllDays - (totleWeek * 2) - endWeekDay;
    }
    lblTime.Text = lastDay.ToString();
}</pre>