Code
 protected void btnCount_Click(object sender, EventArgs e)
    {
        try
        {
            int TotalCount = 0;

            DateTime dtStart = Convert.ToDateTime(tbStartDateTime.Text);
            DateTime dtEnd = Convert.ToDateTime(tbEndDateTime.Text);

            int FirstCount = GetFirstCount(dtStart);
            int LastCount = GetLastCount(dtEnd);
            TotalCount = FirstCount + LastCount;

            DateTime dt1 = Convert.ToDateTime(dtStart.ToShortDateString());  //首日零点
            DateTime dt2 = Convert.ToDateTime(dtEnd.ToShortDateString());  //尾日零点
            TimeSpan ts = dt2 - dt1;
            int days = ts.Days;  //得到跨越的天数

            for (int i = 1; i < days; i++)  //历遍天数(除去首尾日期)
            {
                DateTime dt = dt1.AddDays(i);
                string dayCode = dt.DayOfWeek.GetHashCode().ToString();  //判断是周几
                if ("12345".Contains(dayCode))
                {
                    TotalCount += 8;
                }
                if (dayCode == "6")
                {
                    TotalCount += 4;
                }
            }

            Response.Write("FristDay:" + FirstCount + ",LastDay:" + LastCount + ",OtherDayCount:" + (days - 1) + ",TotalCount:" + TotalCount.ToString());
        }
        catch (Exception exp)
        {
            Response.Write(exp.Message);
            return;
        }
    }
    private int GetFirstCount(DateTime dtStart)
    {
        int hour = dtStart.Hour;
        string dayCode = dtStart.DayOfWeek.GetHashCode().ToString();
        if ("12345".Contains(dayCode))
        {
            if (hour <= 8) return 8;
            else if (hour <= 12) return (17 - hour - 1);
            else if (hour <= 13) return 4;
            else if (hour <= 17) return (17 - hour);
            else return 0;
        }
        if (dayCode == "6")
        {
            if (hour <= 8) return 4;
            else if (hour <= 12) return (12 - hour);
            else return 0;
        }
        return 0;
    }
    private int GetLastCount(DateTime dtEnd)
    {
        int hour = dtEnd.Hour;
        string dayCode = dtEnd.DayOfWeek.GetHashCode().ToString();
        if ("12345".Contains(dayCode))
        {
            if (hour <= 8) return 0;
            else if (hour <= 12) return (hour - 8);
            else if (hour <= 13) return 4;
            else if (hour <= 17) return (hour - 13 + 4);
            else return 0;
        }
        if (dayCode == "6")
        {
            if (hour <= 8) return 0;
            else if (hour <= 12) return (hour - 8);
            else return 4;
        }
        return 0;
    }