Indexers (C# Programming Guide)

Indexers allow instances of a class or struct to be indexed just like arrays. The indexed value can be set or retrieved without explicitly specifying a type or instance member. Indexers resemble properties except that their accessors take parameters.

The following example defines a generic class with simple get and set accessor methods to assign and retrieve values. The Program class creates an instance of this class for storing strings.

using System;

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

   // Define the indexer to allow client code to use [] notation.
   public T this[int i]
   {
      get { return arr[i]; }
      set { arr[i] = value; }
   }
}

class Program
{
   static void Main()
   {
      var stringCollection = new SampleCollection<string>();
      stringCollection[0] = "Hello, World";
      Console.WriteLine(stringCollection[0]);
   }
}
// The example displays the following output:
//       Hello, World.

Note

For more examples, see Related Sections.

Expression Body Definitions

It is common for an indexer's get or set accessor to consist of a single statement that either returns or sets a value. Expression-bodied members provide a simplified syntax to support this scenario. Starting with C# 6, a read-only indexer can be implemented as an expression-bodied member, as the following example shows.

using System;

class SampleCollection<T>
{
   // Declare an array to store the data elements.
   private T[] arr = new T[100];
   int nextIndex = 0;
   
   // Define the indexer to allow client code to use [] notation.
   public T this[int i] => arr[i];
   
   public void Add(T value)
   {
      if (nextIndex >= arr.Length) 
         throw new IndexOutOfRangeException($"The collection can hold only {arr.Length} elements.");
      arr[nextIndex++] = value;
   }
}

class Program
{
   static void Main()
   {
      var stringCollection = new SampleCollection<string>();
      stringCollection.Add("Hello, World");
      System.Console.WriteLine(stringCollection[0]);
   }
}
// The example displays the following output:
//       Hello, World.

Note that => introduces the expression body, and that the get keyword is not used.

Starting with C# 7, both the get and set accessor can be an implemented as expression-bodied members. In this case, both get and set keywords must be used. For example:

using System;

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

   // Define the indexer to allow client code to use [] notation.
   public T this[int i]
   {
      get => arr[i]; 
      set => arr[i] = value; 
   }
}

class Program
{
   static void Main()
   {
      var stringCollection = new SampleCollection<string>();
      stringCollection[0] = "Hello, World.";
      Console.WriteLine(stringCollection[0]);
   }
}
// The example displays the following output:
//       Hello, World.

Indexers Overview

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

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

  • The this keyword is used to define the indexer.

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

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

  • Indexers can be overloaded.

  • Indexers can have more than one formal parameter, for example, when accessing a two-dimensional array.

Related Sections

C# Language Specification

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

See Also

C# Programming Guide
Properties