Programming C# 3.0, 5th Edition

Indexers

Sometimes you may need to access a collection within a class as though the class itself were an array. For example, suppose you create a listbox control named myListBox that contains a list of strings stored in a one-dimensional array, a private member variable named myStrings. A listbox control contains member properties and methods in addition to its array of strings. However, it would be convenient to be able to access the listbox array with an index, just as though the listbox were an array.^[12] For example, such a property would permit statements like the following:

string theFirstString = myListBox[0];
string theLastString = myListBox[Length−1];

An indexer is a C# construct that allows you to access collections contained by a class using the familiar [] syntax of arrays. An indexer is a special kind of property, and includes get and set accessors to specify its behavior.

You declare an indexer property within a class using the following syntax:

type this [type argument]{get; set;}

The return type determines the type of object that will be returned by the indexer, whereas the type argument specifies what kind of argument will be used to index into the collection that contains the target objects. Although it is common to use integers as index values, you can index a collection on other types as well, including strings. You can even provide an indexer with multiple parameters to create a multidimensional array!

The this keyword is a reference to the object in which the indexer ...