VB.NET Language in a Nutshell, Second Edition

The Collection Object

VB.NET implements a special object called the Collection object that acts as a container for objects of all types. In fact, Collection objects can hold other objects, as well as nonobject data.

In some ways, the Collection object is an object-oriented version of the Visual Basic array. It supports the following four methods:

Add

Adds an item to the collection. Along with the data itself, you can specify a key value by which the member can be referenced.

Count

Returns the number of items in the collection.

Item

Retrieves a member from the collection either by its index (or ordinal position in the collection) or by its key (assuming that a key was provided when the item was added to the collection).

Remove

Deletes a member from the collection using the member’s index or key.

For example, the following code defines a collection object named colStates to hold information about U.S. states and then adds two members to it, using the state’s two-letter abbreviation as a key:

Dim colStates As New Collection
colStates.Add("New York", "NY")
colStates.Add("Michigan", "MI")

Like members of an array, the members of a collection can be iterated using the For Each...Next construct. Also like arrays, collection members are accessible by their index value, although the lower bound of a collection object’s index is always 1.

Arrays and collections each have advantages and disadvantages. Some of the advantages of collections over arrays are:

• New collection members can be inserted ...