Indexers, Generic Indexer in C# With Examples

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

The following example defines a generic class with some simple and established accessor methods for assigning and retrieving values. The program class creates an instance of this class to store strings.

Example:

using System;

public class SampleCollection
{
    public string name { get; set; }
    private string[] namecollection;
    public SampleCollection(string name)
    {
        this.name = name;
        this.namecollection = new string[10];
        this.namecollection[0] = "Shahzad";
        this.namecollection[1] = "Shahzad1";
        this.namecollection[2] = "Shahzad2";
    }
    public void show()
    {

        for (int i = 0; i < this.namecollection.Length; i++)
        {
            if (this.namecollection[i] != null)
            {
                Console.WriteLine(this.namecollection[i]);
            }
        }
    }
}
class Program
{
    static void Main(string[] args)
    {
        SampleCollection sampleCollection = new SampleCollection("Shahzad");
        sampleCollection.show();
    }

}

Output:

Shahzad
Shahzad1
Shahzad2

Generic Indexer

In C# Indexer can also be generic. 

Example:

using System;

class ExampleCollection<T>
{
    private T[] array = new T[10];

    public T this[int j]
    {
        get { return array[j]; }
        set { array[j] = value; }
    }
}

class Program
{
    static void Main()
    {
        var MainCollection = new ExampleCollection<string>();
        MainCollection[0] = "Shahzad Hussain";
        MainCollection[1] = "Sabri";
        for (int i = 0; i <= 9; i++)
        {
            if (MainCollection[i] != null)
            {
                Console.WriteLine(MainCollection[i]);
            }
        }
    }
}

Output:

Shahzad Hussain
Sabri

Overload of Indexer

Like functions, indexers can also be overloaded. In C # we can have multiple indexers in a single class. To override an index, you must declare it with multiple parameters, and each parameter must have a different data type. Indexers get overloaded bypassing 2 different kinds of parameters. This is very similar to method overload.

Example:

public class SampleCollection
{
    public string name { get; set; }
    private string[] namecollection;
    public SampleCollection(string name)
    {
        this.name = name;
        this.namecollection = new string[10];
        this.namecollection[0] = "Shahzad";
        this.namecollection[1] = "Shahzad1";
        this.namecollection[2] = "Shahzad2";
    }
    public string this[int index]
    {
        get
        {
            if(index >= 0 && index< this.namecollection.Length)
            {
                return namecollection[index];
            }else
            {
                return "-1";
            }
        }
        set {
            if (index >= 0 && index < this.namecollection.Length)
            {
                namecollection[index] = value;
            }
        }
    }
    public string this[int index,int index1]
    {
        get
        {
            return namecollection[index] + "\n" + namecollection[index1];
        }
    }
    public void show()
    {
        
        for (int i = 0; i<this.namecollection.Length; i++)
        {
            if (this.namecollection[i] != null)
            {
                Console.WriteLine(this.namecollection[i]);
            }
        }
    }
}
class Program
{
     static void Main(string[] args)
    {
        SampleCollection sampleCollection = new SampleCollection("Shahzad");
        //sampleCollection.show();
        Console.WriteLine(sampleCollection[0]);
        Console.WriteLine(sampleCollection[0, 1]);
    }

}

Output:

Shahzad
Shahzad
Shahzad1