Simple understanding and usage of indexers in C#

Indexer is a special class member that allows objects to be accessed in an array-like manner, making the program more intuitive and easier to write.

1. Definition of indexer

Class members in C# can be of any type, including arrays and collections. When a class contains array and collection members, indexers will greatly simplify access to array or collection members.

The way to define an indexer is somewhat similar to defining an attribute. Its general form is as follows:

[修饰符] 数据类型 this[索引类型 index]  
 
{  
    get{//获得属性的代码}                                                   
    set{ //设置属性的代码}  
}

Modifiers include public, protected, private, internal, new, virtual, sealed, override, abstract, extern.

The data type is represented The type of array or collection elements to be accessed.

The indexer type indicates which type of index the indexer uses to access array or collection elements, which can be an integer or a string; this represents the array or collection member that operates this object, which can be simply understood as an index. The name of the indexer, so the indexer cannot have a user-defined name. For example:

class Z  
{  
        //可容纳100个整数的整数集  
        private long[] arr = new long[100];  
        //声明索引器  
        public long this[int index]  
        {  
            get 
            { //检查索引范围  
                if (index < 0 || index >= 100)  
                {  
                    return 0;  
                }  
                else 
                {  
                    return arr[index];  
                }  
            }  
            set 
            {  
                if (!(index < 0 || index >= 100))  
                {  
                    arr[index] = value;  
                }  
            }  
   }

2. Use of indexers

You can access array members of instances of a class through indexers. The operation method is similar to that of arrays. The general form is as follows:

Object name [index]

The data type of the index Must be the same index type as the indexer. For example:

Z  z=new  z();  
z[0]=100;  
z[1]=101;  
Console.WriteLine(z[0]);

means to first create an object z, and then reference the array elements in the object through index.

C# does not limit the type of indexer to integer. For example, you can use strings with indexers. An indexer of this type is implemented by searching for a string within a collection and returning the corresponding value. Since accessors can be overloaded, string and integer versions can coexist.

example:

class DayCollection
{   
      string[] days={"Sun","Mon","Tues","Wed","Thurs","Fri","Sat"};  
      private int GetDay(string testDay)  
     {  
        int i=0;  
        foreach(string day in days)  
          {  
              if(day==testDay)  
                      return i;  
                      i++;  
           }  
         return -1;  
      }  
     public int this[string day]  
      {  
          get{return (GetDay(day))}  
      }  
}  
  
 
static void Main(string[] args)  
{  
     DayCollection week=new DayCollection();  
     Console.WriteLine("Fri:{0}",week["Fri"]);  
     Console.WriteLine("ABC:{0}",week["ABC"]);  
}

Result: Fri:5

ABC:-1

3. Indexer in interface

Indexers can also be declared in interfaces. There are two differences between interface indexers and class indexers. : First, the interface indexer does not use modifiers; second, the interface index only contains the accessor get or set, and no implementation statement. The purpose of the accessor is to indicate whether the indexer is read-write, read-only or write-only. If it is read-write, neither the get nor set accessor can be omitted; if it is read-only, the set accessor is omitted; if it is write-only , omit the get accessor.

For example:

public interface IAddress  
{  
string this[int index]{get;set;}  
string Address{get;set;}  
string Answer();  
}

shows that the declared interface IAddress contains 3 members: an indexer, a property and a method, among which the indexer is readable and writable.

4. Comparison of indexers and attributes

Indexers and attributes are both members of the class and are very similar in syntax. Indexers are generally used in custom collection classes. Using indexers to operate collection objects is as simple as using an array; properties can be used in any custom class, which enhances the flexibility of field members of the class.

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