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Accessing Attributes by Using Reflection (C# and Visual Basic)

The fact that you can define custom attributes and place them in your source code would be of little value without some way of retrieving that information and acting on it. By using reflection, you can retrieve the information that was defined with custom attributes. The key method is GetCustomAttributes, which returns an array of objects that are the run-time equivalents of the source code attributes. This method has several overloaded versions. For more information, see Attribute.

An attribute specification such as:

<Author("P. Ackerman", Version:=1.1)> 
Class SampleClass
    ' P. Ackerman's code goes here... 
End Class

is conceptually equivalent to this:

Dim anonymousAuthorObject As Author = New Author("P. Ackerman")
anonymousAuthorObject.version = 1.1

However, the code is not executed until SampleClass is queried for attributes. Calling GetCustomAttributes on SampleClass causes an Author object to be constructed and initialized as above. If the class has other attributes, other attribute objects are constructed similarly. GetCustomAttributes then returns the Author object and any other attribute objects in an array. You can then iterate over this array, determine what attributes were applied based on the type of each array element, and extract information from the attribute objects.

Here is a complete example. A custom attribute is defined, applied to several entities, and retrieved via reflection.

' Multiuse attribute
<System.AttributeUsage(System.AttributeTargets.Class Or 
                       System.AttributeTargets.Struct, 
                       AllowMultiple:=True)> 
Public Class Author
    Inherits System.Attribute
    Private name As String 
    Public version As Double 
    Sub New(ByVal authorName As String)
        name = authorName

        ' Default value
        version = 1.0
    End Sub 

    Function GetName() As String 
        Return name
    End Function         
End Class 

' Class with the Author attribute
<Author("P. Ackerman")> 
Public Class FirstClass
End Class 

' Class without the Author attribute 
Public Class SecondClass
End Class 

' Class with multiple Author attributes.
<Author("P. Ackerman"), Author("R. Koch", Version:=2.0)> 
Public Class ThirdClass
End Class 

Class TestAuthorAttribute
    Sub Main()
        PrintAuthorInfo(GetType(FirstClass))
        PrintAuthorInfo(GetType(SecondClass))
        PrintAuthorInfo(GetType(ThirdClass))
    End Sub 

    Private Shared Sub PrintAuthorInfo(ByVal t As System.Type)
        System.Console.WriteLine("Author information for {0}", t)

        ' Using reflection 
        Dim attrs() As System.Attribute = System.Attribute.GetCustomAttributes(t)

        ' Displaying output 
        For Each attr In attrs
            Dim a As Author = CType(attr, Author)
            System.Console.WriteLine("   {0}, version {1:f}", a.GetName(), a.version)
        Next             
    End Sub 

    ' Output: 
    '   Author information for FirstClass 
    '     P. Ackerman, version 1.00 
    ' Author information for SecondClass 
    ' Author information for ThirdClass 
    '  R. Koch, version 2.00 
    '  P. Ackerman, version 1.00 

End Class
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