Interface member with "this" keyword

Question

While going through our client's code, I came across below interface in C#, which is having a member with "this" keyword.

 public interface ISettings
{
    string this[string key] { get; }
}

I am not aware of any such pattern or practice where interface member name starts with "this". To understand more, I checked the implementation of this interface, however still not able to figure out its purpose.

internal class SettingsManager : ISettings
{
    public string this[string key]
    {
        get { return ConfigurationManager.AppSettings[key]; }
    }
   ...
   ...
}

And here is the caller code:

public static class Utility
{
   public static ISettings Handler { get; set; }

    public static string Get(string key, string defaultValue)
    {
        var result = Handler[key];

        return Is.EmptyString(result) ? defaultValue : result;
    }
}

Unfortunately, I am not able to debug this code to see the things live. But very curious about it. If the implemented code is finally returning a string, then what is the use of "this" keyword out there?

Answer 1

It enables you to do things like:

SettingsManager settings = new SettingsManager();
var setting = settings["my setting"];

A common use is with the List<T> class.

It has the definition:

public class List<T> : IList<T>, ICollection<T>, IList, ICollection, IReadOnlyList<T>, IReadOnlyCollection<T>, IEnumerable<T>, IEnumerable
{
    // ....

    public T this[int index] { get; set; }

    // ....
}

This allows you to 'index' the internal values in a similar way to an array.

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace test
{
    static class Program
    {
        static void Main()
        {
            List<string> myStrings = new List<string>();

            myStrings.Add("abc");
            myStrings.Add("def");

            Console.WriteLine(myStrings[0]); // outputs: "abc"
            Console.WriteLine(myStrings[1]); // outputs: "def"

            Console.Read();
        }
    }
}

Answer 2

They are indexers, allowing to access your class like an array, in the example your provided you see the usage in this line:

var result = Handler[key];