An indexer allows an instance of a class or struct to be indexed as an array. If the user will define an indexer for a class, then the class will behave like a virtual array. Array access operator i.e ([ ]) is used to access the instance of the class which uses an indexer. A user can retrieve or set the indexed value without pointing an instance or a type member. Indexers are almost similar to the Properties. The main difference between Indexers and Properties is that the accessors of the Indexers will take parameters.

Syntax:

[access_modifier] [return_type] this [argument_list]
{
  get 
  {
     // get block code
  }
  set 
  {
    // set block code
  }

}

In the above syntax:

• access_modifier: It can be public, private, protected or internal.
• return_type: It can be any valid C# type.
• this: It is the keyword which points to the object of the current class.
• argument_list: This specifies the parameter list of the indexer.
• get{ } and set { }: These are the accessors.

Example:

// C# program to illustrate the Indexer
using System;
 
// class declaration
class IndexerCreation
{
 
    // class members
    private string[] val = new string[3];
     
    // Indexer declaration
    // public - access modifier
    // string - the return type of the Indexer
      // this - is the keyword having a parameters list
    public string this[int index]
    {
 
        // get Accessor
        // retrieving the values
        // stored in val[] array
        // of strings
        get
        {
 
            return val[index];
        }
 
        // set Accessor
        // setting the value at
        // passed index of val
        set
        {
 
            // value keyword is used
            // to define the value
            // being assigned by the
            // set indexer.
            val[index] = value;
        }
    }
}
 
// Driver Class
class main {
     
    // Main Method
    public static void Main() {
         
        // creating an object of parent class which
        // acts as primary address for using Indexer
        IndexerCreation ic = new IndexerCreation();
 
        // Inserting values in ic[]
        // Here we are using the object
        // of class as an array
        ic[0] = "C";
        ic[1] = "CPP";
        ic[2] = "CSHARP";
 
        Console.Write("Printing values stored in objects used as arrays\n");
         
        // printing values
        Console.WriteLine("First value = {0}", ic[0]);
        Console.WriteLine("Second value = {0}", ic[1]);
        Console.WriteLine("Third value = {0}", ic[2]);
     
    }
}

Output:

Printing values stored in objects used as arrays
First value = C
Second value = CPP
Third value = CSHARP