一个索引器的笑问题

Question

using System;
using System.Collections.Generic;
using System.Text;namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Album al = new Album(5);
            photo ph1 = new photo();
            ph1.Name = "杭州";
            photo ph2 = new photo();
            ph2.Name = "北海";
            photo p = al["杭州"];
            Console.WriteLine(p.Name.ToString());
            Console.ReadLine();
        }
    }    class Album
    {
        photo[] phs;        internal photo[] Phs
        {
            get { return phs; }
            set { phs = value; }
        }
        public Album(int count)
        {
            phs = new photo[count];
        }
        public photo this[string name]
        {
            get 
            {
                photo ph = null;
                for (int i = 0; i < phs.Length; i++)
                {
                    if (phs[i].Name == name)
                    {
                        ph = phs[i];
                    }
                }
                return ph;
            }
        }
    }    class photo
    {
        string _name;        public string Name
        {
            get { return _name; }
            set { _name = value; }
        }
    }
}在这段索引器代码中“if (phs[i].Name == name)”会报错说没有实例化对象是怎么回事啊？我明明已经实例化了啊

Answer 1

你只是new了一个数组，数组里的元素仍然是null啊

Answer 2

那我在
  static void Main(string[] args) 
        { 
            Album al = new Album(5); 
            photo ph1 = new photo(); 
            ph1.Name = "杭州"; 
            photo ph2 = new photo(); 
            ph2.Name = "北海"; 
            photo p = al["杭州"]; 
            Console.WriteLine(p.Name.ToString()); 
            Console.ReadLine(); 
        } 
里面加上
              al.Phs[0] = ph1;
            al.Phs[1] = ph2;
变成
 static void Main(string[] args)
        {
            Album al = new Album(5);
            photo ph1 = new photo();
            ph1.Name = "杭州";
            photo ph2 = new photo();
            ph2.Name = "北海";
            al.Phs[0] = ph1;
            al.Phs[1] = ph2;
            photo p = al["杭州"];
            Console.WriteLine(p.Name.ToString());
            Console.ReadLine();
        }
还是不行啊。

Answer 3

当然不行了，你那个循环到2的时候依然数组Phs[2~5]依然是null

Answer 4

索引器可以这样写public photo this[string name]
        {
            get 
            {
                foreach (photo ph in phs)
                {
                    if (ph != null)
                    {
                        if (ph.Name == name)
                        {
                            return ph;
                        }
                    }
                }
                return null;
            }
        }

Answer 5

建议按照下面的写法：using System; 
using System.Collections.Generic; 
using System.Text; 
using System.IO;namespace ConsoleApplication1 
{     class Program 
    { 
        static void Main(string[] args) 
        { 
            Album al = new Album();
            photo ph1 = new photo();
            ph1.Name = "杭州";
            photo ph2 = new photo();
            ph2.Name = "北海";
            al.AddPhoto(ph1);
            al.AddPhoto(ph2);
            photo p = al["杭州州"];
            if (p != null)
            {
                Console.WriteLine(p.Name.ToString());
            }
            else
            {
                Console.WriteLine("对象不存在");
            }            Console.ReadLine();        } 
    } 
    class Album
    {
        List<photo> phs=new List<photo>();
        public Album()
        {
        }        public void AddPhoto(photo ph)
        {
            FindPhoto f = new FindPhoto(ph.Name);
            if (!this.phs.Exists(new Predicate<photo>(f.hasPhoto)))
            {
                this.phs.Add(ph);
            }
        }                public photo this[string name]
        {
            get
            {
                FindPhoto f = new FindPhoto(name);
                if (this.phs.Exists(new Predicate<photo>(f.hasPhoto)))
                {
                    return this.phs.Find(new Predicate<photo>(f.hasPhoto));
                }
                return null;
            }
        }
    }    class FindPhoto
    { 
       private string photoName=String.Empty;
        public FindPhoto(string pName)
        {
            this.photoName = pName;
        }        public bool hasPhoto(photo ph)
        {
            if (ph.Name.Equals(this.photoName))
            {
                return true;
            }
            else
            {
                return false;
            }
        }
    }    class photo
    {
        string _name;        public string Name
        {
            get { return _name; }
            set { _name = value; }
        }
    } 
}