VB.NET Language in a Nutshell, Second Edition

Defining a Custom Attribute

An attribute is merely a class that inherits from System.Attribute, which makes it very easy to implement a custom attribute. In this section, we’ll build a custom attribute called <DeveloperNote>, which allows a developer to add assorted information (the developer’s name, the date, a comment, and whether a code modification was a response to a bug) to code. The steps are as follows:

1. Define a public class that inherits from System.Attribute or another attribute class derived from System.Attribute. For example:

   Public Class DeveloperNoteAttribute
    Inherits System.Attribute

   Note that, by convention, the name of the class ends with the substring “Attribute”.

2. Apply the <AttributeUsage> attribute, which defines the language elements to which the custom attribute can be applied, to the class (as shown in the following code fragment). The attribute’s only required argument is one of the following members of the AttributeTargets enumeration:

   All

   Assembly

   Class

   Constructor

   Delegate

   Enum

   Event

   Field

   Interface

   Method

   Module

   Parameter

   Property

   ReturnValue

   Struct

   If an attribute applies to multiple programming elements, but not all elements, the relevant constants can be ORed together. In the case of our <DeveloperNote> attribute, we want the attribute to apply to all program elements. In addition, we want to make the <DeveloperNote> attribute extensible through inheritance, so we set the <AttributeTarget> attribute’s Inherited argument to True. Finally, ...