从接口转换为基础类型

审查 earlier question 因此,我开始考虑这样的情况:一个类将一个值(如集合)公开为一个由该值的类型实现的接口。在下面的代码示例中,我使用一个列表,并将该列表公开为IEnumerable。

通过IEnumerable接口公开列表定义了只枚举列表而不修改列表的意图。但是,由于实例可以重新转换回列表,因此当然可以修改列表本身。

我还在示例中包含一个方法版本,该版本通过在每次调用该方法时将列表项引用复制到新列表来防止修改,从而防止对基础列表的更改。

请注意,我理解所示行为是预期行为。我并不是说这种行为是错误的,只是它确实允许使用表达意图以外的功能。也许我给接口赋予了太多的意义——把它看作一个功能约束。思想?

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

namespace TypeCastTest
{
    class Program
    {
        static void Main(string[] args)
        {
            // Demonstrate casting situation
            Automobile castAuto = new Automobile();

            List<string> doorNamesCast = (List<string>)castAuto.GetDoorNamesUsingCast();

            doorNamesCast.Add("Spare Tire");

            // Would prefer this prints 4 names, 
            // actually prints 5 because IEnumerable<string>
            // was cast back to List<string>, exposing the 
            // Add method of the underlying List object
            // Since the list was cast to IEnumerable before being
            // returned, the expressed intent is that calling code
            // should only be able to enumerate over the collection,
            // not modify it.
            foreach (string doorName in castAuto.GetDoorNamesUsingCast())
            {
                Console.WriteLine(doorName);
            }

            Console.WriteLine();

            // --------------------------------------

            // Demonstrate casting defense
            Automobile copyAuto = new Automobile();

            List<string> doorNamesCopy = (List<string>)copyAuto.GetDoorNamesUsingCopy();

            doorNamesCopy.Add("Spare Tire");

            // This returns only 4 names,  
            // because the IEnumerable<string> that is 
            // returned is from a copied List<string>, so 
            // calling the Add method of the List object does
            // not modify the underlying collection
            foreach (string doorName in copyAuto.GetDoorNamesUsingCopy())
            {
                Console.WriteLine(doorName);
            }

            Console.ReadLine();
        }
    }

    public class Automobile
    {
        private List<string> doors = new List<string>();
        public Automobile()
        {
            doors.Add("Driver Front");
            doors.Add("Passenger Front");
            doors.Add("Driver Rear");
            doors.Add("Passenger Rear");
        }

        public IEnumerable<string> GetDoorNamesUsingCopy()
        {
            return new List<string>(doors).AsEnumerable<string>();
        }
        public IEnumerable<string> GetDoorNamesUsingCast()
        {
            return doors.AsEnumerable<string>();
        }
    }

}