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Indexer Example

Let's look at some places where indexers make the most sense. I'll start with the list box example I've already used. As mentioned, from a conceptual standpoint, a list box is simply a list, or an array of strings to be displayed. In the following example, I've declared a class called MyListBox that contains an indexer to set and retrieve strings through an ArrayList object. (The ArrayList class is a .NET Framework class used to store a collection of objects.) -

using System;
using System.Collections;

class MyListBox

{
    protected ArrayList data = new ArrayList();

    public object this[int idx]
    {
        get
        {
            if (idx > -1 && idx < data.Count)
            {
                return (data[idx]);
            }
            else
            {
                // Possibly throw an exception here.
                return null;
            }
        }
        set
        {
            if (idx > -1 && idx < data.Count)
            {
                data[idx] = value;
            }
            else if (idx == data.Count)
            {
                data.Add(value);
            }
            else
            {
                // Possibly throw an exception here.
            }
        }
    }
}

class Indexers1App
{
    public static void Main()
    {
        MyListBox lbx = new MyListBox();
        lbx[0] = "foo";
        lbx[1] = "bar";
        lbx[2] = "baz";
        Console.WriteLine("{0} {1} {2}",
                           lbx[0], lbx[1], lbx[2]);
    }
}

Notice in this example that I check for out-of-bounds errors in the indexing of the data. This is not technically tied to indexers because, as I mentioned, indexers pertain only to how the class's client can use the object as an array and have nothing to do with the internal representation of the data. However, when learning a new language feature, it helps to see practical usage of a feature rather than only its syntax. So, in both the indexer's getter and setter methods, I validate the index value being passed with the data being stored in the class's ArrayList member. I personally would probably choose to throw exceptions in the cases where the index value being passed can't be resolved. However, that's a personal choice-your error handling might differ. The point is that you need to indicate failure to the client in cases where an invalid index has been passed.

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