The Visual Basic .NET Programming Language

The language defines keywords such as Public , ByRef , and Overridable that can be applied to declarations to modify their meaning. These predefined modifiers can be augmented by user -defined attributes that can store extra declarative information about types and type members . For example, attributes can be used to declare that a type or type member is obsolete and should no longer be used.

<Obsolete("Use NewSquare class instead.")> _ Public Class Square ... End Class Public Class NewSquare ... <Obsolete("Use NewMove method instead.")> _ Public Sub Move(ByVal x As Integer, ByVal y As Integer) ... End Sub Public Sub NewMove(ByVal x As Integer, ByVal y As Integer, _ ByVal Refresh As Boolean) ... End Sub End Class

Attributes are defined by declaring a class that inherits from the type System.Attribute . A System.AttributeUsageAttribute attribute must also be applied to the class. For example, the following type defines an Author attribute that can be used to name the author of a type.

<AttributeUsage(AttributeTargets.All)> _ Class AuthorAttribute Inherits Attribute Public Name As String Public Company As String Public Sub New(ByVal Name As String) Me.Name = Name End Sub End Class

The parameters to the constructor of a user-defined attribute define the arguments that must be supplied when the attribute is specified on a type or type member. Fields and properties in the user-defined attribute can also be assigned to. For example:

<Author("John Doe", Company := "MegaCorp")> _ Module Test <Author("Jane Doe", Company := "Acme")> _ Sub Main() End Sub End Module

To retrieve attributes, the Framework reflection classes can be used.

<Author("John Doe", Company := "MegaCorp")> _ Class TestClass End Class Module Test Sub Main() Dim T As Type = GetType(TestClass) Dim AuthAttribute As AuthorAttribute AuthAttribute = CType(T.GetCustomAttributes(False)(0), _ AuthorAttribute) Console.WriteLine(AuthAttribute.Name) Console.WriteLine(AuthAttribute.Company) End Sub End Module