The indexer in C#(C#中的索引器)

Indexers allow instances of a class or struct to be indexed just like arrays. Indexers resemble properties except that their accessors take parameters.

In the following example, a generic class is defined and provided with simple get and set accessor methods as a means of assigning and retrieving values. The Program class creates an instance of this class for storing strings.

class SampleCollection<T>
{
    // Declare an array to store the data elements.
    private T[] arr = new T[100];

    // Define the indexer, which will allow client code
    // to use [] notation on the class instance itself.
    // (See line 2 of code in Main below.)        
    public T this[int i]
    {
        // This indexer is very simple, and just returns or sets
        // the corresponding element from the internal array.
        get
        {
            return arr[i];
        }
        set
        {
            arr[i] = value;
        }
    }
}

// This class shows how client code uses the indexer.
class Program
{
    static void Main(string[] args)
    {
        // Declare an instance of the SampleCollection type.
        SampleCollection<string> stringCollection = new SampleCollection<string>();

        // Use [] notation on the type.
        stringCollection[0] = "Hello, World";
        System.Console.WriteLine(stringCollection[0]);
    }
}

 

Indexers Overview

  1. Indexers enable objects to be indexed in a similar manner to arrays.

  2. A get accessor returns a value. A set accessor assigns a value.

  3. The this keyword is used to define the indexers.

  4. The value keyword is used to define the value being assigned by the set indexer.

  5. Indexers do not have to be indexed by an integer value; it is up to you how to define the specific look-up mechanism.

  6. Indexers can be overloaded.

  7. Indexers can have more than one formal parameter, for example, when accessing a two-dimensional array. Below is simple example:

//declaration:
    class Mlass
    {    
        private int[,] mint = new int[100,100];
        public int this[int i, int j]    
        {
            get        
            {
                return mint[i, j];
            }
            set
            {
                mint[i, j] = value;
            }
        }
    }

//usage:
    Mlass m = new Mlass();
    m[2, 2] = 3;
    Console.WriteLine(m[2, 2]);

 

Addition

  1. Just like any other class members, indexers can also participate in inheritance. A base class indexer is inherited to the derived class.

  2. A Base class indexer can be polymorphicaly overridden in a Derived class. But remember that the modifiers like virtual, override etc are using at property level, not at accessor level.

  3. An indexer inside a class can be declared as abstract by using the keyword abstract. Remember that an abstract indexer in a class carries no code at all. The get/set accessors are simply represented with a semicolon. In the derived class we must implement both set and get assessors.

　　If the abstract class contains only set accessor, we can implement only set in the derived class.

 

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