using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            DateTime dt1 = DateTime.Now;
            DateTime dt2 = DateTime.Now.AddDays(100);
            int n = 0;
            for (DateTime dt = dt1; dt <= dt2; dt = dt.AddDays(1))
            {
                if (dt.DayOfWeek != DayOfWeek.Sunday && dt.DayOfWeek != DayOfWeek.Saturday) n++;
            }
            Console.WriteLine(n);
        }
    }
}

------解决方案--------------------

你可以用DateTimePicker控件，用户选取值后在它的Value属性中。
TextBox也可以，用Parse方法解析。