Question

This is the main class

 class Program
{
    static void Main(string[] args)
    {
        MyArray fruit = new MyArray(-2, 1);
        fruit[-2] = "Apple";
        fruit[-1] = "Orange";
        fruit[0] = "Banana";
        fruit[1] = "Blackcurrant";
        Console.WriteLine(fruit[-1]);       // Outputs "Orange"
        Console.WriteLine(fruit[0]);        // Outputs "Banana"
        Console.WriteLine(fruit[-1,0]);     // Output "O"
        Console.ReadLine();
    }
}

and here is the class for the indexer:

class MyArray
{
    int _lowerBound;
    int _upperBound;
    string[] _items;

    public MyArray(int lowerBound, int upperBound)
    {
        _lowerBound = lowerBound;
        _upperBound = upperBound;
        _items = new string[1 + upperBound - lowerBound];
    }

    public string this[int index]
    {
        get { return _items[index - _lowerBound]; }
        set { _items[index - _lowerBound] = value; }

    }

    public string this[int word, int position]
    {
        get { return _items[word - _lowerBound].Substring(position, 1); }
    }
}

So, multiDimensional indexer in defined in the ‘Class MyArray’

I am not able to understand how this is working when we pass ‘-1’ as the value of ‘word’ to the indexer. The value for ‘_lowerbound’ is ‘-2’. So it means, the value for the return should be _items[-1 – (-2)], which makes it _items[1].

But actually it is pointing to -1 index of fruits i.e. Orange.

Please clear my doubt.

Answer 1

This is you private array _items:

   0     1     2     3
┌────┐┌────┐┌────┐┌────┐
│ Ap ││ Or ││ Ba ││ Bl │          
└────┘└────┘└────┘└────┘  

This is how it appears using the indexer:

  -2    -1     0     1
┌────┐┌────┐┌────┐┌────┐
│ Ap ││ Or ││ Ba ││ Bl │          
└────┘└────┘└────┘└────┘  

The value for _lowerbound is -2.

So it means, the value for the return should be _items[-1 - (-2)], which makes it _items[1].

That’s correct.

But actually it is pointing to -1 index of fruits i.e. Orange.

That’s wrong. It is pointing to _items[1], i.e. Orange.

Indexers are a simple way to permit a class to be used like an array. Internally, you can manage the values presented in any fashion you wish.

_items is a zero-based array with the same length as the created MyArray object.

The indexer is just interpreting the index number supplied and map it against this underlying array.