Indexer with custom types in C Sharp (C#)

Introduction

Indexers let you access objects like arrays using custom keys. Using custom types as keys makes your code clearer and more organized.

You want to access elements in a collection using a special key type, not just numbers.

You have a class that stores data identified by a custom object, like a coordinate or a name.

You want to make your class easier to use by allowing bracket [] access with your own key type.

Syntax

C Sharp (C#)

public ReturnType this[CustomKeyType key] {
    get { /* return value for key */ }
    set { /* set value for key */ }
}

The keyword this defines the indexer.

You can define get and set to read and write values.

Examples

Indexer using a custom Coordinate type as key.

C Sharp (C#)

public string this[Coordinate coord] {
    get { return data[coord]; }
    set { data[coord] = value; }
}

Indexer using a Person object as key to store ages.

C Sharp (C#)

public int this[Person person] {
    get { return ages[person]; }
    set { ages[person] = value; }
}

Sample Program

This program creates a Grid class with an indexer that uses a Coordinate record as the key. It stores and retrieves strings at specific coordinates.

C Sharp (C#)

using System;
using System.Collections.Generic;

public record Coordinate(int X, int Y);

public class Grid {
    private Dictionary<Coordinate, string> data = new();

    public string this[Coordinate coord] {
        get => data.ContainsKey(coord) ? data[coord] : "Empty";
        set => data[coord] = value;
    }
}

class Program {
    static void Main() {
        var grid = new Grid();
        grid[new Coordinate(0, 0)] = "Start";
        grid[new Coordinate(1, 2)] = "Tree";

        Console.WriteLine(grid[new Coordinate(0, 0)]);
        Console.WriteLine(grid[new Coordinate(1, 2)]);
        Console.WriteLine(grid[new Coordinate(2, 2)]); // no value set
    }
}

Important Notes

Make sure your custom key type implements equality correctly (like a record) so dictionary lookups work.

Indexers make your class feel like a collection with easy access syntax.

Summary

Indexers let you use bracket [] access on your classes.

You can use any custom type as the key for an indexer.

Implement get and set to control how values are accessed and stored.