using System;

public class Program

{

    public static void Main()

    {

        string s = "hello";

        Console.WriteLine (s[0]); // 'h'

        Console.WriteLine (s[3]); // 'l'

        s = null;

        Console.WriteLine (s?[0]); // Writes nothing; no error.

    }

}

class Sentence

{

string[] words = "The quick brown fox".Split();

public string this [int wordNum]

{

get { return words [wordNum]; }

set { words [wordNum] = value; }

    }

}