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AttributeUsage (Visual Basic)

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Determines how a custom attribute class can be used. AttributeUsage is an attribute that can be applied to custom attribute definitions to control how the new attribute can be applied. The default settings look like this when applied explicitly:

<System.AttributeUsage(System.AttributeTargets.All,
                   AllowMultiple:=False,
                   Inherited:=True)>
Class NewAttribute
    Inherits System.Attribute
End Class

In this example, the NewAttribute class can be applied to any attribute-able code entity, but can be applied only once to each entity. It is inherited by derived classes when applied to a base class.

The AllowMultiple and Inherited arguments are optional, so this code has the same effect:

<System.AttributeUsage(System.AttributeTargets.All)>
Class NewAttribute
    Inherits System.Attribute
End Class

The first AttributeUsage argument must be one or more elements of the AttributeTargets enumeration. Multiple target types can be linked together with the OR operator, like this:

<AttributeUsage(AttributeTargets.Property Or AttributeTargets.Field)>
Class NewPropertyOrFieldAttribute
    Inherits Attribute
End Class

If the AllowMultiple argument is set to true, then the resulting attribute can be applied more than once to a single entity, like this:

<AttributeUsage(AttributeTargets.Class, AllowMultiple:=True)>
Class MultiUseAttr
    Inherits Attribute
End Class

<MultiUseAttr(), MultiUseAttr()>
Class Class1
End Class

In this case MultiUseAttr can be applied repeatedly because AllowMultiple is set to true. Both formats shown for applying multiple attributes are valid.

If Inherited is set to false, then the attribute is not inherited by classes that are derived from a class that is attributed. For example:

<AttributeUsage(AttributeTargets.Class, Inherited:=False)>
Class Attr1
    Inherits Attribute
End Class

<Attr1()>
Class BClass

End Class

Class DClass
    Inherits BClass
End Class

In this case Attr1 is not applied to DClass via inheritance.

Remarks

The AttributeUsage attribute is a single-use attribute--it cannot be applied more than once to the same class. AttributeUsage is an alias for AttributeUsageAttribute.

For more information, see Accessing Attributes by Using Reflection (Visual Basic).

Example

The following example demonstrates the effect of the Inherited and AllowMultiple arguments to the AttributeUsage attribute, and how the custom attributes applied to a class can be enumerated.

' Create some custom attributes:
<AttributeUsage(System.AttributeTargets.Class, Inherited:=False)>
Class A1
    Inherits System.Attribute
End Class

<AttributeUsage(System.AttributeTargets.Class)>
Class A2
    Inherits System.Attribute
End Class

<AttributeUsage(System.AttributeTargets.Class, AllowMultiple:=True)>
Class A3
    Inherits System.Attribute
End Class

' Apply custom attributes to classes:
<A1(), A2()>
Class BaseClass

End Class

<A3(), A3()>
Class DerivedClass
    Inherits BaseClass
End Class

Public Class TestAttributeUsage
    Sub Main()
        Dim b As New BaseClass
        Dim d As New DerivedClass
        ' Display custom attributes for each class.
        Console.WriteLine("Attributes on Base Class:")
        Dim attrs() As Object = b.GetType().GetCustomAttributes(True)

        For Each attr In attrs
            Console.WriteLine(attr)
        Next

        Console.WriteLine("Attributes on Derived Class:")
        attrs = d.GetType().GetCustomAttributes(True)
        For Each attr In attrs
            Console.WriteLine(attr)
        Next
    End Sub
End Class

Sample Output

Attributes on Base Class:
A1
A2
Attributes on Derived Class:
A3
A3
A2

See also