The book Illustrated C# 7, Fifth Edition by Daniel Solis and Cal Schrotenboer published by Apress has a discussion of indexers on page 162. Indexers are similar to how arrays work.

string str = null;
Console.WriteLine(str?[0]); // Writes nothing; no error.
Console.WriteLine(str[0]); // NullReferenceException
To write an indexer, define a property called this, specifying the arguments in square brackets.
class Sentence
{
    string[] words = "The quick brown fox".Split(); //field
    public Sentence() { }  // default constructor
    public Sentence(string str) // constructor
            { words = str.Split(); }
    public int Length  // property
            { get { return words.Length; } }
    public string this[int wordNum] // indexer
    {
        get { return words[wordNum]; }
        set { words[wordNum] = value; }
    }
}
static void Main(string[] args)
{
    string s = "hello world";
    Console.WriteLine(s[0]); // 'h' zero-based
    Console.WriteLine(s[5]); // ' '
    string str = null;
    Console.WriteLine(str?[0]); // Writes nothing; no error.
    // Console.WriteLine(str[0]); // NullReferenceException
 
    Sentence sen = new Sentence();
    Console.WriteLine(sen[1]); // quick
    sen[3] = "wildebeest";  // replace the 4th word
    Console.WriteLine(sen[3]); // wildebeest
    for (int i=0;i<sen.Length;i++) { Console.Write(sen[i] + "|"); }
    // now use our constructor to use our sentence
    Sentence sent = new Sentence("The sleeping black cat");
    Console.WriteLine(sent[1]);  // sleeping
}