Indexers in C# allow objects to be indexed like arrays. This is useful when your object represents a collection of values and you want to access its elements using the array-like object[index] syntax.

What Is an Indexer?

An indexer lets you use the array index syntax ([]) on a class or struct to access data stored inside it — just like an array or a list.

It’s defined using the this keyword followed by an index parameter.

Syntax of an Indexer

class ClassName
{
    private string[] data = new string[10];

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

Example: Creating and Using an Indexer

using System;

class StudentNames
{
    private string[] names = new string[5];

    // Define the indexer
    public string this[int index]
    {
        get { return names[index]; }
        set { names[index] = value; }
    }
}

class Program
{
    static void Main()
    {
        StudentNames students = new StudentNames();

        students[0] = "Alice";
        students[1] = "Bob";

        Console.WriteLine(students[0]); // Output: Alice
        Console.WriteLine(students[1]); // Output: Bob
    }
}

Key Points

1. Indexers use the this keyword.
2. You can define get and/or set accessors.
3. You can overload indexers by changing the number or types of parameters.
4. Indexers can be read-only, write-only, or read/write.

Indexers vs Properties

Example: String-based Indexer

You can also create indexers that accept strings or other data types.

class BookLibrary
{
    private Dictionary<string, string> books = new();

    public string this[string title]
    {
        get { return books.ContainsKey(title) ? books[title] : "Not found"; }
        set { books[title] = value; }
    }
}

Usage 

var library = new BookLibrary();
library["C#"] = "Learn C# in 21 Days";
Console.WriteLine(library["C#"]); // Output: Learn C# in 21 Days

Access Modifiers in Indexers

You can set different access modifiers for the get and set parts:

public string this[int index]
{
    get { return data[index]; }
    private set { data[index] = value; }
}

Summary

Read More - 

• Overview
• Using Indexers
• Indexers in Interfaces
• Comparison Between Properties and Indexers