Detailed introduction to indexers in C#

Indexer(Indexer) is a new type of class member introduced in C#, which makes the Objects can be referenced as conveniently and intuitively as arrays. Indexers are very similar to Attributes, but indexers can have parameter lists and can only act on instance objects, not directly on classes. A class that defines an indexer allows you to access it like an array [ ] OperatorAccess members of a class. (Of course there are many advanced applications, such as mapping arrays through indexers, etc.)

##Indexers allow instances of a class or structure to be indexed just like an array. Indexers are similar to properties, The difference is that their values ​​Function takes parameters. Indexers allow instances of classes or structures to be indexed in the same way as arrays. Indexers are similar to properties, except that indexer access takes parameters.

Indexer Overview

• Use an indexer to index objects in a manner similar to an array.

• #get The value function returns the value.

  set The value function assigns a value.

• #This keyword is used to define the indexer.

• ##The value keyword is used to define the value assigned by the set indexer.

• The indexer does not have to index based on

  integer values; it is up to you to define the specific lookup mechanism.

• Indexers can be

  overloaded.

• The indexer can have multiple formal parameters, such as when accessing a

  two-dimensional array.

Indexer and array comparison:

(1)The index value (Index) type of the indexer is not restricted

(2)The indexer allows overloading

(3)The indexer is not a variable

indexer Differences from attributes

(1) Attributes are identified by names, and indexers are identified by functions

(2) Indexers can be overloaded, but attributes cannot

(3) Indexers cannot be declared as static, attributes can be

A simple indexer example

using System;
using System.Collections;

public class IndexerClass
{
    private string[] name = new string[2];

    //索引器必须以this关键字定义，其实这个this就是类实例化之后的对象
    public string this[int index]
    {
        //实现索引器的get方法
        get
        {
            if (index < 2)
            {
                return name[index];
            }
            return null;
        }

        //实现索引器的set方法
        set
        {
            if (index < 2)
            {
                name[index] = value;
            }
        }
    }
}
public class Test
{
    static void Main()
    {
        //索引器的使用
        IndexerClass Indexer = new IndexerClass();
        //“=”号右边对索引器赋值，其实就是调用其set方法
        Indexer[0] = "张三";
       Indexer[1] = "李四";
        //输出索引器的值，其实就是调用其get方法
        Console.WriteLine(Indexer[0]);
       Console.WriteLine(Indexer[1]);
    }
}

Use string as a subscript to access the indexer

public class IndexerClass
{
    //用string作为索引器下标的时候，要用Hashtable
    private Hashtable name = new Hashtable();

    //索引器必须以this关键字定义，其实这个this就是类实例化之后的对象
    public string this[string index]
    {
        get { return name[index].ToString(); 
        set { name.Add(index, value); }
    }
}
public class Test
{
    static void Main()
    {
        IndexerClass Indexer = new IndexerClass();
        Indexer["A0001"] = "张三";
        Indexer["A0002"] = "李四";
        Console.WriteLine(Indexer["A0001"]);
        Console.WriteLine(Indexer["A0002"]);
    }
}

Overloading of indexer

public class IndexerClass
{
    private Hashtable name = new Hashtable();

    //1：通过key存取Values
    public string this[int index]
    {
        get { return name[index].ToString(); }
        set { name.Add(index, value); }
    }

    //2：通过Values存取key
    public int this[string aName]
    {
        get
        {
            //Hashtable中实际存放的是DictionaryEntry(字典)类型，如果要遍历一个Hashtable，就需要使用到DictionaryEntry
            foreach(DictionaryEntry d in name)
            {
                if (d.Value.ToString() == aName)
                {
                    return Convert.ToInt32(d.Key);
                }
            }
            return -1;
        }
        set
        {
            name.Add(value, aName);
        }
    }
}
public class Test
{
    static void Main()
    {
        IndexerClass Indexer = new IndexerClass();

        //第一种索引器的使用
        Indexer[1] = "张三";//set访问器的使用
        Indexer[2] = "李四";
       Console.WriteLine("编号为1的名字：" + Indexer[1]);//get访问器的使用
        Console.WriteLine("编号为2的名字：" + Indexer[2]);

        Console.WriteLine();
        //第二种索引器的使用
        Console.WriteLine("张三的编号是：" + Indexer["张三"]);//get访问器的使用
        Console.WriteLine("李四的编号是：" + Indexer["李四"]);
       Indexer["王五"] = 3;//set访问器的使用
        Console.WriteLine("王五的编号是：" + Indexer["王五"]);
    }
}

Multi-parameter indexer

using System;
using System.Collections;

//入职信息类
public class EntrantInfo
{
    //姓名、编号、部门
    private string name;
    private int number;
    private string department;
    public EntrantInfo()
    {

    }
    public EntrantInfo(string name, int num, string department)
    {
        this.name = name;
        this.number = num;
        this.department = department;
    }

    public string Name
    {
        get { return name; }
        set { name = value; }
    }

    public int Num
    {
        get { return number; }
        set { number = value; }
    }

    public string Department
    {
        get { return department; }
        set { department = value; }
    }
}

//声明一个类EntrantInfo的索引器
public class IndexerForEntrantInfo
{
    private ArrayList ArrLst;//用于存放EntrantInfo类
    public IndexerForEntrantInfo()
    {
        ArrLst = new ArrayList();
    }

    //声明一个索引器：以名字和编号查找存取部门信息
    public string this[string name, int num]
    {
        get
        {
            foreach (EntrantInfo en in ArrLst)
            {
                if (en.Name == name && en.Num == num)
                {
                    return en.Department;
                }
            }
            return null;
        }
        set
        {
            //new关键字：C#规定，实例化一个类或者调用类的构造函数时，必须使用new关键
            ArrLst.Add(new EntrantInfo(name, num, value));
        }
    }

    //声明一个索引器：以编号查找名字和部门
    public ArrayList this[int num]
    {
        get
        {
            ArrayList temp = new ArrayList();
            foreach (EntrantInfo en in ArrLst)
            {
                if (en.Num == num)
                {
                    temp.Add(en);
                }
            }
            return temp;
        }
    }

    //还可以声明多个版本的索引器...
}

public class Test
{
    static void Main()
    {
        IndexerForEntrantInfo Info = new IndexerForEntrantInfo();
        //this[string name, int num]的使用
        Info["张三", 101] = "人事部";
        Info["李四", 102] = "行政部";
        Console.WriteLine(Info["张三", 101]);
        Console.WriteLine(Info["李四", 102]);

        Console.WriteLine();

        //this[int num]的使用
        foreach (EntrantInfo en in Info[102])
        {
            Console.WriteLine(en.Name);
            Console.WriteLine(en.Department);
        }
    }
}

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