090923 T 一个对象一对多关联的问题

编程时遇到对象关联API的设计问题，感觉可能这种api的设计方案本身就有问题，所以目前还未有解决方案。
问题如下：

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

namespace ConsoleApplication
{
    /// <summary>
    /// 在NotImplement的方法/属性中填入相应内容，以保证程序不会抛出异常。
    /// (就是：Parent属性，AddChild方法，RemoveChild方法。)
    /// 
    /// 不能修改其它代码。
    /// </summary>
    class Program
    {
        static void Main(string[] args)
        {
            Parent parentA = new Parent();
            Parent parentB = new Parent();
            Child child = new Child();

            parentA.AddChild(child);

            child.Parent = parentB;
            Assert(parentA.Children.Count == 0, parentB.Children.Count == 1, child.Parent == parentB);

            parentA.AddChild(child);
            Assert(parentB.Children.Count == 0, parentA.Children.Count == 1, child.Parent == parentA);

            child.Parent = null;
            Assert(parentB.Children.Count == 0, parentA.Children.Count == 0, child.Parent == null);
        }

        static void Assert(params bool[] values)
        {
            foreach (var value in values)
            {
                if (value == false)
                {
                    throw new Exception();
                }
            }
        }
    }
    class Parent
    {
        private List<Child> _children;

        public Parent()
        {
            this._children = new List<Child>();
        }

        public IList<Child> Children
        {
            get
            {
                return new System.Collections.ObjectModel.ReadOnlyCollection<Child>(this._children);
            }
        }
        public void AddChild(Child child)
        {
            throw new NotImplementedException();
        }
        public void RemoveChild(Child child)
        {
            throw new NotImplementedException();
        }

    }
    class Child
    {
        private Parent _parent;

        public Parent Parent
        {
            get
            {
                throw new NotImplementedException();
            }
            set
            {
                throw new NotImplementedException();
            }
        }
    }
}