Indexers in C#

A feature in the C# language that I have noticed that many developers, even some with years of C# experience, have missed is that it is possible to create Indexers.
With Indexers you are able to use square brackets to access specific elements of a class, just like you would in an Array or a List:

items = new List<int> { 1, 2, 3, 4 };
var first = items[0];

Assume that you define an interface for a certain ability that a class can have. That ability being that it should be possible to set and get elements of the class using square brackets. You may define the interface like this:

namespace Indexer
{
    public interface IIndexable<T>
    {
       T this[int index] { get; set; }
    }
}

And a class can implement the interface like this:

using System.Collections.Generic;
 
namespace Indexer
{
    internal class Indexable : IIndexable<int>
    {
        private readonly IList<int> items;
 
        public Indexable()
        {
            items = new List<int> { 1, 2, 3, 4 };
        }
 
        public int this[int index]
        {
            get => items[index];
            set => items[index] = value;
        }
    }
}

Now, a user of the Indexable class will be able to get and set individual elements of the Indexable using the same syntax as for an Array or List:

using System;
 
namespace Indexer
{
    public class IndexerUser 
    {
        public void UseIndexable()
        {
            var indexable = new Indexable();
            Console.WriteLine(indexable[0]);
 
            indexable[3] = 8;
            Console.WriteLine(indexable[3]);
        }
    }
}

Of course, in this simple example you would be better off with just a simple List. But for larger, more complex classes using Indexers comes in handy from time to time.