Programming Visual Basic .NET, Second Edition

Indexers and the Default Property

Some classes contain their own internal collection. For example, you might write your own School class that would contain, as a private member variable, a collection of the Students enrolled in the school. You might then want to access the School class as if it were an array of Students. To do so, you would use the default property, which will allow you to write:

Dim joe As Student = mySchool(5)

accessing the sixth element in mySchool’s internal collection!

As another 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 if the listbox were an array. For example, such a property would permit statements like the following:

Dim theFirstString As String = myListBox(0)

You implement this with the default property. Each class can have one default property, designated with the Default keyword. It is common to use the property name Item for the default property, but that is not required.

You can retrieve the default property with or without the property name. The following two code lines both retrieve the default property (which in this case, is called Item); the first uses the name, the second doesn’t:

Dim theFirstString As String = myListBox.Item(0) Dim ...