Indexers

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

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]

    {

        get

        {

            // This indexer is very simple, and just returns or sets

            // the corresponding element from the internal array.

            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]);

    }

}

// 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 indexers.
• 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.

Yield

yield is a contextual keyword used in iterator methods in C#. Basically, it has two use cases:

• yield return obj; returns the next item in the sequence.
• yield break; stops returning sequence elements (this happens automatically if control reaches the end of the iterator method body).