Understanding Objects and Collections

As you’ve seen in earlier chapters of this book, you can produce powerful applications in Microsoft Access without programming. However, when you need a more sophisticated application, writing Visual Basic code gives you a greater degree of control over what your application does. When you program in Visual Basic, you work with objects that correspond to different aspects of your database. Collections are sets of objects of the same type. Programming with objects and collections gives you added flexibility in that you can design your application to respond to user actions and input in a customized way.

Objects available to you in Microsoft Access come from four different sources:

• Microsoft Access provides objects that you use to display your data, such as Form, Report, and Control objects.
• Microsoft DAO provides Data Access Objects (DAO), such as the TableDef and QueryDef objects, which determine the structure of your database and which you can use to manipulate data in Visual Basic.
• Visual Basic provides objects that give you more flexibility in programming, such as the Debug object and the Err object.
• Microsoft Office provides objects that you can use to customize the interface of your application, such as the CommandBar and FileSearch objects.

Many of the objects that you work with in Visual Basic correspond to specific parts of your Microsoft Access database. For example, the Microsoft Access Form and Report objects correspond to your forms and reports, while the DAO TableDef and QueryDef objects correspond to your tables and queries. The Microsoft Office CommandBar object corresponds to the toolbars, menu bars, menus, and shortcut menus you see in Microsoft Access.

Other objects you may work with in Visual Basic are more abstract. For example, the Visual Basic Err object contains information about errors that occur while Visual Basic code is running, rather than corresponding to a specific part of your database.

Even though the objects available to you in Microsoft Access come from several different sources and perform different functions, you can work with them in similar ways. You’ll find that the concepts you need to understand in order to program with one object apply to most objects.

For example, every object has properties and methods associated with it. Once you understand how to work with one object’s properties and methods, you can use those same concepts to work with any object’s properties and methods. You use properties to determine or change some characteristic of an object. For example, you use a form’s Visible property to determine whether or not the form is visible to the user. You use methods to perform a particular operation on an object. For example, the Repaint method completes any pending screen updates on a specified form.

Organization of Objects and Collections

Each application that supplies objects to Microsoft Access provides an object library. The object library contains information about an application’s objects and their properties and methods. Microsoft Access includes four built-in object libraries, which are described in the following sections.

Microsoft Access objects and DAO objects are organized in object hierarchies. In an object hierarchy, certain objects contain other objects and collections. A collection is a special type of object that is actually a set of all objects of a given type. When you refer to a collection in code, you are referring to all the objects in the set. For example, the Microsoft Access Application object contains a Forms collection, which contains individual Form objects. A Form object contains a Controls collection, which in turn contains individual Control objects. The following illustration shows this relationship.

You can think of a collection as an array of objects that’s already declared by Microsoft Access. Collections, like arrays, have elements, and you refer to the objects in a collection as elements of the collection. You can refer to an element of a collection by its name or by its position within the collection.

See Also   For more information on arrays, see “Arrays” in Chapter 4, “Working with Variables, Data Types, and Constants.”

The following sections describe the four object libraries included in Microsoft Access.

Microsoft Access Objects

You’re probably already familiar with some of the objects in the Microsoft Access 8.0 object library. Microsoft Access Form, Report, and Control objects correspond to your forms, reports, and controls. You use these objects to control the way you display the data in your database. The Application object and the Reference object make it easier to work with objects in other applications. The Module object gives you control over the Visual Basic modules in your database. You use the DoCmd object to include macro actions in your code and the Screen object to refer to the active object on the screen.

The following table describes the objects provided by the Microsoft Access 8.0 object library.