Indexers (this[])

Part of the Object Oriented Programming section of Coddy's C# journey — lesson 37 of 70.

Indexers let you access elements in your class using array-like bracket notation. Instead of calling a method like collection.GetItem(0), you can write the more intuitive collection[0].

You define an indexer using the this keyword with square brackets:

public class Sentence
{
    private string[] words;
    
    public Sentence(string text)
    {
        words = text.Split(' ');
    }
    
    public string this[int index]
    {
        get { return words[index]; }
        set { words[index] = value; }
    }
}

Now you can access words naturally:

Sentence s = new Sentence("Hello World");
Console.WriteLine(s[0]);  // Hello
s[1] = "C#";
Console.WriteLine(s[1]);  // C#

Indexers aren't limited to integers. You can use any type as the index parameter, making them useful for dictionary-like access:

public string this[string key]
{
    get { return dictionary[key]; }
    set { dictionary[key] = value; }
}

Like properties, indexers can be read-only (omit the setter) or include validation logic in their accessors. They're ideal when your class wraps a collection or when bracket notation makes the code more readable.

Challenge

Easy

Let's build a Playlist class that lets you access songs using intuitive bracket notation, just like working with an array. You'll create a music playlist where songs can be retrieved and updated by their position using an indexer.

You'll organize your code across two files:

Playlist.cs: Create a Playlist class in the Music namespace that wraps an internal collection of songs and provides array-like access through an indexer. Your playlist should have:
- A private string array to store song titles
- A constructor that accepts a comma-separated string of song titles and splits it into the internal array
- An indexer using this[int index] that allows both getting and setting songs at a specific position
- A read-only property Count that returns the number of songs in the playlist
- A method GetAllSongs() that returns all songs joined by " | "
Program.cs: In your main file, create a playlist from input, then use the indexer to access and modify songs. Demonstrate both reading songs by index and updating a song at a specific position.

You will receive three inputs:

A comma-separated list of song titles (e.g., Song A,Song B,Song C)
An index to retrieve (integer)
A new song title to replace the song at that same index

Print the output in this format:

Total songs: {Count}
Song at index {index}: {song title}
After update: {GetAllSongs()}

For example, if the inputs are Rock Anthem,Jazz Vibes,Pop Hit, 1, and Blues Classic, the output should be:

Total songs: 3
Song at index 1: Jazz Vibes
After update: Rock Anthem | Blues Classic | Pop Hit

Notice how the indexer lets you access playlist[1] to get "Jazz Vibes" and then assign playlist[1] = "Blues Classic" to update it - much more natural than calling methods like GetSongAt(1) or SetSongAt(1, "Blues Classic")!

Cheat sheet

Indexers allow you to access elements in your class using array-like bracket notation (collection[0]) instead of method calls.

Define an indexer using the this keyword with square brackets:

public class Sentence
{
    private string[] words;
    
    public Sentence(string text)
    {
        words = text.Split(' ');
    }
    
    public string this[int index]
    {
        get { return words[index]; }
        set { words[index] = value; }
    }
}

Usage example:

Sentence s = new Sentence("Hello World");
Console.WriteLine(s[0]);  // Hello
s[1] = "C#";
Console.WriteLine(s[1]);  // C#

Indexers can use any type as the index parameter, not just integers:

public string this[string key]
{
    get { return dictionary[key]; }
    set { dictionary[key] = value; }
}

Like properties, indexers can be read-only (omit the setter) or include validation logic in their accessors.

Try it yourself

using System;
using Music;

class Program
{
    public static void Main(string[] args)
    {
        // Read inputs
        string songList = Console.ReadLine();
        int index = Convert.ToInt32(Console.ReadLine());
        string newSong = Console.ReadLine();

        // TODO: Create a Playlist object from the songList

        // TODO: Print the total number of songs using the Count property

        // TODO: Print the song at the given index using the indexer

        // TODO: Update the song at the given index using the indexer

        // TODO: Print all songs after the update using GetAllSongs()
    }
}