8.2 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 : -
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". -
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 OR ed 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, we want to allow the application of multiple attributes to the same program element; hence, we want to set the AllowMultiple argument to True as well. In view of this setting, our code should look as follows: <AttributeUsage(AttributeTargets.All, _ Inherited:=True, _ AllowMultiple:=True)> _ Public Class DeveloperNoteAttribute Inherits System.Attribute -
Create the class constructor (the New subroutine), which is called when the attribute is applied to a particular language element. The class constructor defines the attribute's required or positional arguments. At a minimum, we'll want a developer to record his or her name, a comment, and the date. Our constructor appears as follows: Public Sub New(Name As String, Comment As String, _ DateRecorded As String) MyBase.New( ) strName = Name strComment = Comment datDate = CDate(DateRecorded) End Sub Note that the date is passed to the constructor as a String type. There is some restriction on the data types that can be used as attribute parameters. Parameters can be any integral data type (Byte, Short, Integer, Long) or floating point data type (Single and Double), as well as Char, String, Boolean, an enumerated type, or System.Type. Thus, Date, Decimal, Object, and structured types cannot be used as parameters. Each required parameter also corresponds to a class property or field. These parameters are added to the class in the next step. -
Declare properties or fields. The attribute's public properties and fields correspond both to parameters required by the class constructor and to optional parameters supplied when the attribute is applied to a language element. In the case of our attribute, we'll want properties that correspond to each attribute, as well as an additional Bugs property that indicates whether or not the comment corresponds to a code modification that resulted from a bug. The code is: Public Property Name As String Get Return strName End Get Set strName = Value End Set End Property Public Property Comment As String Get Return strComment End Get Set strComment = Value End Set End Property Public Property DateRecorded As Date Get Return datDate End Get Set datDate = Value End Set End Property Public Property Bug As Boolean Get Return blnBug End Get Set blnBug = Value End Set End Property The complete code for the attribute class is shown in Example 8-1. Example 8-1. The DeveloperNoteAttribute attribute class Option Strict On Imports System Namespace Extensions.CustomAttributes <AttributeUsage(AttributeTargets.All, _ Inherited:=True, _ AllowMultiple:=True)> _ Public Class DeveloperNoteAttribute Inherits System.Attribute Protected strName, strComment As String Protected blnBug As Boolean Protected datDate As Date Public Sub New(Name As String, Comment As String, DateRecorded As String) MyBase.New( ) strName = Name strComment = Comment datDate = CDate(DateRecorded) End Sub Public Property Name As String Get Return strName End Get Set strName = Value End Set End Property Public Property Comment As String Get Return strComment End Get Set strComment = Value End Set End Property Public Property DateRecorded As Date Get Return datDate End Get Set datDate = Value End Set End Property Public Property Bug As Boolean Get Return blnBug End Get Set blnBug = Value End Set End Property End Class End Namespace |