Explore the various benefits of the Collection object in VB6

by Peter Aitken | Aug 01, 2005 7:00:00 AM

Tags: Peter Aitken, benefit, Microsoft Visual Basic 6.0, Visual Basic Tips Newsletter

Takeaway: VB6 makes use of collections in many ways, such as keeping track of controls on a form and system printers. Learn how to create a Collection object, as well as how to use the Add method to add items to the collection.

A collection is an ordered set of items that you can refer to as a unit. VB6 makes use of collections in many ways, such as keeping track of controls on a form and system printers. VB6 also provides a generic Collection object for you to use in your programs. What makes the Collection object so useful is that the items it contains can be essentially anything�variables, literal numbers or text, objects, and so on. And, they don't have to be the same data type.

To create a Collection object, you use the usual VB syntax:

Dim col As New Collection

Then, you use the Add method to add items to the collection:

col.Add item, key, before, after

Here's a description of each of the arguments:

Item is the item being added. This argument is required.
Key is an optional string that identifies the item. Key must be unique within the collection.
Before is an optional number that specifies the position of the new item in the collection as the index of the existing item before which the new item is to be placed.
After is an optional number that specifies the position of the new item in the collection as the index of the existing item after which the new item is to be placed.

You wouldn't use the before and after arguments at the same time. If you omit both arguments, the new item is placed at the end of the collection. If you use one of these arguments, it must refer to an existing member of the collection or else an error will occur.

To retrieve an item from a collection, use the Item method:

col.Item(index)

The index argument is either a number specifying the position of the item in the collection, or the key string that was specified when the item was added. If the index argument doesn't match an item in the collection, an error occurs. Note that the numerical index is one-based; in other words, it runs from 1 to the number of items in the collection. Since Item is the default method, you can use the shorthand syntax like this:

col(index)

To delete an item, use the Remove method. Its syntax is exactly the same as Item. If you remove an item that isn't the last item, other items move up to fill in the space of the deleted item.

The Collection object has one property, Count. It returns the number of items in the collection.

Collections can be useful in various situations. For instance, suppose your program lets the user create one or more child windows. You can use a collection to keep track of them and then destroy them as needed. First, create the collection:

Dim col As New Collection

Then, when each window is created, add it to the collection:

Dim f As New Form2
col.Add f
f.Show

When it's time to close all the windows, loop through the collection, like this:

Dim i As Integer
Dim f As Form
For i = 1 To col.Count
    Set f = col(i)
    f.Visible = False
    Set f = Nothing
Next

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