C# Indexers

An indexer allows us to access instances of a class using an index just like an array.

Define C# Indexer

In C#, we define an indexer just like properties using this keyword followed by [] index notation. For example,

 public int this[int index] { get { return val[index]; } set { val[index] = value; } }

Here,

• public - access modifier
• int - return type of indexer
• this - indicates we are defining indexer in current class
• int index - access values using integer index position
• get - ,method that returns values
• set - method that assigns values

Example: C# indexer

 using System; class Program { // declare an array to store elements private string[] studentName = new string[10]; 
 // define an indexer public string this[int index] { get { // return value of stored at studentName array return studentName[index]; } set { // assigns value to studentName studentName[index] = value; } }
 public static void Main() { // create instance of Program class Program obj = new Program(); // insert values in obj[] using indexer i.e index position obj[0] = "Harry"; obj[1] = "Ron"; obj[2] = "Hermoine"; Console.WriteLine("First element in obj: " + obj[0]); Console.WriteLine("Second element in obj: " + obj[1]); } }

Output

 First element in obj: Harry Second element in obj: Ron

In the above example, notice the code

 // declare an array to store elements private string[] studentName = new string[10];

Here, we have declared the studentName array of size 10.

Then we have defined an indexer of string type. Notice the code below,

 // define an indexer public string this[int index] { get { // return value stored at studentName array return studentName[index]; } set { // assigns value to studentName studentName[index] = value; } }

Here, the set method assigns values to studentName using index. And the get method returns values stored at studentName.

Notice the code,

 // create instance of Program class Program obj = new Program(); // insert values in obj[] using indexer i.e index position obj[0] = "Harry"; obj[1] = "Ron"; obj[2] = "Hermoine";

Here, we have used obj (instance of the Program class) like the studentName array.

 // insert value to studentName array when indexer is not used obj.studentName[0] = "Harry";

Generic Indexer in C#

In C#, we can also use indexers with a generic class. For example,

 using System; class EmployeeInfo<T> { // declare an array to store elements private T[] employee = new T[50]; 
 // define an indexer, // T indicates return type of indexer is generic public T this[int index]
 { get { // return value of stored at studentName array return employee[index]; } set { // assigns value to studentName employee[index] = value; } } } class Program { public static void Main() { // create instance of EmployeeInfo class of int type EmployeeInfo<int> Id = new EmployeeInfo<int>(); // insert integer values in Id[] using indexer i.e index position Id[0] = 3; Id[1] = 23; Id[2] = 10; Console.WriteLine("First element in Id object: " + Id[0]); // create instance of EmployeeInfo class of string type EmployeeInfo<string> Name = new EmployeeInfo<string>(); // insert string values in Name[] using indexer i.e index position Name[0] = "Taylor"; Name[1] = "Selena"; Name[2] = "Joe"; Console.WriteLine("First element in Name object: " + Name[0]); } }

Output

 First element in Id object: 3 First element in Name object: Taylor

In the above example, notice the code,

 public T this[int index] { get { // return value of stored at studentName array return employee[index]; } set { // assigns value to studentName employee[index] = value; } }

Here, we have defined an indexer with return type T which indicates it can return any generic data type.

Also, we have created an instance of the EmployeeInfo class of type int and string. Notice the code,

 // create instance of EmployeeInfo class of int type EmployeeInfo<int> Id = new EmployeeInfo<int>(); // create instance of EmployeeInfo class of string type EmployeeInfo<string> Name = new EmployeeInfo<string>();

Here, the two instances Id and Name are used like an array using index. Notice the code,

 // insert integer values in Id[] using indexer i.e index position Id[0] = 3; // insert string values in Name[] using indexer i.e index position Name[0] = "Taylor";