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:
is conceptually equivalent to this:
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
