c#屬性和索引器

1、屬性
所謂屬性其實就是特殊的類別成員，它實作了對私有類別域的受控存取。在C#語言中有兩種屬性方法，其一是get，透過它可以傳回私有域的值，其二是set，透過它就可以設定私有域的值。比如說，以下面的程式碼為例，建立學生姓名屬性，控制對name欄位的受控存取：

using System;

public class Student
{
    private string name;
    /// <summary>
    /// 定义学生的姓名属性
    /// </summary>
    public string Name
    {
        get { return name; }
        set { name = value; }
    }
}


class Program
{
    static void Main(string[] args)
    {
        Student student = new Student();
        student.Name = "Jeff Wong";
        Console.WriteLine(student.Name);
        Console.Read();
    }
}

2、索引器
簡單說來，所謂索引器就是一類特殊的屬性，透過它們你就可以像引用數組一樣引用自己的類別。顯然，這項功能在創建集合類別的場合特別有用，而在其他某些情況下，例如處理大型檔案或抽象某些有限資源等，能讓類別具有類似陣列的行為當然也是非常有用的。例如，上例中，我們假設一個班級有若干個學生，建構索引器就可以很方便地呼叫：

using System;
using System.Collections.Generic;

public class Student
{
    public List<Student> listStudents = new List<Student>();

    /// <summary>
    /// 构建索引器
    /// </summary>
    /// <param name="i"></param>
    /// <returns></returns>
    public Student this[int i]
    {
        get { return listStudents[i]; }
        set { listStudents[i] = value; }
    }

    private string name;
    /// <summary>
    /// 属性
    /// </summary>
    public string Name
    {
        get { return name; }
        set { name = value; }
    }
    public Student(string name)
    {
        this.name = name;
    }
    public Student()
    {
        this.listStudents.Add(new Student("jeff wong"));
        this.listStudents.Add(new Student("jeffery zhao"));
        this.listStudents.Add(new Student("terry lee"));
        this.listStudents.Add(new Student("dudu"));
    }
}


class Program
{
    static void Main(string[] args)
    {
        Student student = new Student();
        int num = student.listStudents.Count;
        Console.WriteLine("All the students:");
        for (int i = 0; i < num; i++)
        {
            Console.WriteLine(student[i].Name); //通过索引器，取所有学生名
        }

        //设置索引器的值
        student[0].Name = "jeff";
        Console.WriteLine("After modified,all the students:");
        for (int i = 0; i < num; i++)
        {
            Console.WriteLine(student[i].Name); 
        }

        Console.Read();
    }
}

上面程式碼中，我們看到索引器的存取器帶一個參數（參數為整數），其實可以建構多個參數的索引器。也以上述代碼為例，我們要根據學生學號和姓名得到學生的考試總分，修改後代碼如下：

using System;
using System.Collections.Generic;

public class Student
{
    public List<Student> listStudents = new List<Student>();

    public Student this[int i,string name]
    {
        get
        {
            foreach (Student stu in listStudents.ToArray())
            {
                if (stu.sid == i && stu.name == name) //按照学号和姓名取出学生
                {
                    return stu;
                }
            }
            return null;
        }
        set { listStudents[i] = value; }
    }

    private int sid; //学号
    public int Sid
    {
        get { return sid; }
        set { sid = value; }
    }
    private string name;//姓名
    public string Name
    {
        get { return name; }
        set { name = value; }
    }
    private int score; //总分
    public int Score
    {
        get { return score; }
        set { score = value; }
    }
    public Student(int sid, string name, int score)
    {
        this.sid = sid;
        this.name = name;
        this.score = score;
    }
    public Student()
    {
        this.listStudents.Add(new Student(1, "jeff wong", 375));
        this.listStudents.Add(new Student(2,"jeffery zhao",450));
        this.listStudents.Add(new Student(3,"terry lee",400));
        this.listStudents.Add(new Student(4,"dudu",500));
    }
}


class Program
{
    static void Main(string[] args)
    {
        Student student = new Student();
        Student stu = student[1, "jeff wong"];
        Console.WriteLine("student number:" + stu.Sid + ",name:" + stu.Name + ",score:" + stu.Score);

        Console.Read();
    }
}

3、總結：
<1>、

屬性的定義：
訪問修飾符傳回型別屬性名稱

｛
      get｛語句集合｝
          get｛語句集合｝
            get｛語句集合｝
       set｛語句集合｝

｝

<2>、

索引器使得物件可依與陣列相似的方法索引。
this 關鍵字用於定義索引器。
get 訪問器傳回值。 set 存取器指派值。

value 關鍵字用於定義由 set 索引器指派的值。

索引器不必根據整數值進行索引，由你決定如何定義特定的查找機制。
索引器可被重載。

<3>、屬性和索引器的主要區別： 

a、類別的每個屬性都必須擁有唯一的名稱，而類別裡定義的每一個索引器都必須擁有唯一的簽章（signature）或參數清單（這樣就可以實作索引器重載）。 
b、屬性可以是static（靜態的）而索引器則必須是實例成員。
<4>、索引器重載實例：

using System;
using System.Collections.Generic;

public class Student
{
    public List<Student> listStudents = new List<Student>();

    public Student this[int i,string name]
    {
        get
        {
            foreach (Student stu in listStudents.ToArray())
            {
                if (stu.sid == i && stu.name == name) //按照学号和姓名取出学生
                {
                    return stu;
                }
            }
            return null;
        }
        set { listStudents[i] = value; }
    }

    /// <summary>
    /// 索引器重载
    /// </summary>
    /// <param name="i"></param>
    /// <returns></returns>
    public Student this[int i] //i从0开始
    {
        get { return listStudents[i]; }
        set { listStudents[i] = value; }
    }

    private int sid; //学号
    public int Sid
    {
        get { return sid; }
        set { sid = value; }
    }
    private string name;//姓名
    public string Name
    {
        get { return name; }
        set { name = value; }
    }
    private int score; //总分
    public int Score
    {
        get { return score; }
        set { score = value; }
    }
    public Student(int sid, string name, int score)
    {
        this.sid = sid;
        this.name = name;
        this.score = score;
    }
    public Student()
    {
        this.listStudents.Add(new Student(1, "jeff wong", 375));
        this.listStudents.Add(new Student(2,"jeffery zhao",450));
        this.listStudents.Add(new Student(3,"terry lee",400));
        this.listStudents.Add(new Student(4,"dudu",500));
    }
}


class Program
{
    static void Main(string[] args)
    {
        Student student = new Student();
        Student stu = student[1, "jeff wong"];
        Console.WriteLine("student number:" + stu.Sid + ",name:" + stu.Name + ",score:" + stu.Score);
      
        Console.WriteLine("all the students:");

        for (int i = 0; i < student.listStudents.Count; i++)
        {
            Console.WriteLine("student number:" + student[i].Sid + ",name:" + student[i].Name + ",score:" + student[i].Score);
        }

        Console.Read();
    }
}

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