using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication2
{
    public class one
    {
        public virtual void print()
        { }
    }
    public class two : one
    {
        public override void print()
        {
            Console.Write("two方法");
        }
    }
    public class three : one
    {
        public override void print()
        {
            Console.Write("three方法");
        }
    }
    public class im
    {
        public static one returnclass(string name)
        {
            switch (name)
            {
                case "two":
                    return new two();
                case "three":
                    return new three();
            }
           // return null;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            one adm = im.returnclass("three");
            adm.print();
        }
    }
}


请问一下。。c#中的工厂模式是不是一定要使用抽象类来定义呢？还有。在returnclass函数中。为何将return null注释后，提示错误呢？为何不能注释掉呢？