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Working with Custom Collections

In addition to the collections built into the Access and other object libraries, you can create custom collections. Custom collections are similar to arrays, but they offer several advantages:

1. Collections are dynamically allocated. They take up memory based only on what's in them at a given time. This is different from arrays, whose size must be either predefined or redimensioned at runtime. When you redimension an array, Access actually makes a copy of the array in memory, taking up substantial resources. By using custom collections, you can avoid that.

2. A collection always knows how many elements it has, and elements can easily be added and removed.

3. Each element of a collection can contain a different type of data.

4. Elements can be added into any element of a collection.

Although collections are very powerful and provide several advantages, it is important that you be aware of their disadvantages, which are as follows:

1. Every item in a collection is stored as a variant.

2. Although the capability to store a different type of data in each element of a collection can be an advantage, it can also be a disadvantage. If you attempt to treat each item in the collection the same (for example, by accessing the same property in each element of the collection), your code might render an error.

You might wonder why collections are covered in this section. A common use of collections is to house instances of custom objects. An example of such a use is covered in the section of this chapter titled "Using a Collection to Manipulate Multiple Instances of the FileInformation Class."

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