Enhancing Classes with Attributes

Class-modifying attributes are something we discussed way back in Chapter 1, "Introducing .NET," and they have nothing to do with GDI+. I just wanted to refresh your memory, because they will be used in this chapter's project code.

Class- or member-modifying attributes appear just before the definition of the class or member, and within angle brackets. This code attaches the ObsoleteAttribute attribute to the SomeOldClass class.

<ObsoleteAttribute> _ Class SomeOldClass    ...class details here... End Class 

(You can leave the "Attribute" part of an attribute's name off if the first part of the name doesn't conflict with any Visual Basic keyword.) Attributes appear as metadata in the final compiled assembly, and they are used by classes and applications that, by design, extract meaning from specific attributes. In this chapter's code, we'll make use of the PropertyGrid control, the control that implements the Properties panel within the Visual Studio development environment, and is often used to modify Form and Control properties. This control is available for your use in your own applications. To use it, assign a class instance to the control's SelectedObject property. Then, magically, all of the object's properties appear in the controls list of properties.

Nice as this is, it's not always desirable. Your object may have properties that should not be displayed. The PropertyGrid control is designed to be generic; it doesn't know about your object's needs, so it doesn't know which properties to exclude. That is, it doesn't know until you tell it through attributes. By adding specific attributes to your class's properties, you tell the PropertyGrid control how to treat members of your object. For instance, the BrowsableAttribute attribute tells the PropertyGrid to include (True) or exclude (False) the property.

<Browsable(False)> _ Public Property SecretProperty() As String... 

I'll supply additional details about this when we use the PropertyGrid control later in this chapter.