c# properties and indexers

1. Attributes
The so-called attributes are actually special class members, which implement controlled access to private class fields. There are two attribute methods in the C# language. One is get, through which the value of the private field can be returned, and the second is set, through which the value of the private field can be set. For example, take the following code as an example to create a student name attribute and control controlled access to the name field:

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. Indexer
Simply put, the so-called indexer is a special type of attribute, through which you can You can reference your own class just like an array. Obviously, this feature is particularly useful when creating collection classes, but in other situations, such as processing large files or abstracting certain limited resources, it is of course also very useful to have array-like behavior in a class. For example, in the above example, we assume that there are several students in a class. Building the indexer can be easily called:

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();
    }
}

In the above code, we see that the accessor of the indexer takes a parameter (the parameter is an integer). In fact, Indexers can be constructed with multiple parameters. Taking the above code as an example, we want to get the student's total test score based on the student's student number and name. The modified code is as follows:

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. Summary:
<1>,

Definition of attributes:
Access modification Character return type attribute name

｛
       get｛ statement collection ｝
        set ｛ statement collection ｝
｝

indexer definition:

access modifier return type this [parameter type parameter...]
｛
  get ｝｝

e set ｛statement collection｝

& & & lt; 2 & gt;

indexes so that the object can index in a similar method to the array.
This keyword is used to define the indexer.
get accessor return value. The set accessor assigns a value.
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.
<3> The main difference between attributes and indexers:
a. Each attribute of a class must have a unique name, and each indexer defined in the class must have a unique signature or parameter list (This enables indexer reloading).

b. Properties can be static (static) and indexers must be instance members.

<4>, Indexer overloading example:

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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