AttributeUsage(C# 和 Visual Basic)

确定可以如何使用自定义特性类。AttributeUsage 是一个可应用于自定义特性定义,以便控制如何应用新特性的特性。显式应用时默认设置如下所示:

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

[System.AttributeUsage(System.AttributeTargets.All,
                   AllowMultiple = false,
                   Inherited = true)]
class NewAttribute : System.Attribute { }

在本例中,NewAttribute 类可应用于任何支持特性的代码实体,但是对每个实体只可应用一次。当应用于基类时,它可由派生类继承。

AllowMultiple 和 Inherited 参数是可选的,所以此代码具有相同的效果:

<System.AttributeUsage(System.AttributeTargets.All)> 
Class NewAttribute
    Inherits System.Attribute
End Class
[System.AttributeUsage(System.AttributeTargets.All)]
class NewAttribute : System.Attribute { }

第一个 AttributeUsage 参数必须是 AttributeTargets 枚举的一个或多个元素。多个目标类型可使用 OR 运算符链接在一起,如下例所示:

Imports System


...


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


...


[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field)]
class NewPropertyOrFieldAttribute : Attribute { }

如果 AllowMultiple 参数设置为 true,则返回特性可对单个实体应用多次,如下例所示:

Imports System


...


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

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

using System;


...


[AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
class MultiUseAttr : Attribute { }

[MultiUseAttr]
[MultiUseAttr]
class Class1 { }

[MultiUseAttr, MultiUseAttr]
class Class2 { }

在这种情况下,由于 AllowMultiple 设置为 true,MultiUseAttr 可反复应用。所示的应用多个特性的这两种格式均有效。

如果 Inherited 设置为 false,则该特性不由从特性化的类派生的类继承。例如:

Imports System


...


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

<Attr1()> 
Class BClass

End Class  

Class DClass
    Inherits BClass
End Class
using System;


...


[AttributeUsage(AttributeTargets.Class, Inherited = false)]
class Attr1 : Attribute { }

[Attr1]
class BClass { }

class DClass : BClass { }

在这种情况下,Attr1 不通过继承应用于 DClass。

备注

AttributeUsage 特性是一个单用途特性,它无法对相同的类应用多次。AttributeUsage 是 AttributeUsageAttribute 的别名。

有关更多信息,请参见 使用反射访问特性(C# 和 Visual Basic)

示例

下面的示例将阐释 Inherited 参数和 AllowMultiple 参数对 AttributeUsage 特性的效果,以及如何才能枚举应用于类的自定义特性。

Imports System


...


    ' 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

using System;


...


// Create some custom attributes:
[AttributeUsage(System.AttributeTargets.Class, Inherited = false)]
class A1 : System.Attribute { }

[AttributeUsage(System.AttributeTargets.Class)]
class A2 : System.Attribute { }

[AttributeUsage(System.AttributeTargets.Class, AllowMultiple = true)]
class A3 : System.Attribute { }

// Apply custom attributes to classes:
[A1, A2]
class BaseClass { }

[A3, A3]
class DerivedClass : BaseClass { }

public class TestAttributeUsage
{
    static void Main()
    {
        BaseClass b = new BaseClass();
        DerivedClass d = new DerivedClass();

        // Display custom attributes for each class.
        Console.WriteLine("Attributes on Base Class:");
        object[] attrs = b.GetType().GetCustomAttributes(true);
        foreach (Attribute attr in attrs)
        {
            Console.WriteLine(attr);
        }

        Console.WriteLine("Attributes on Derived Class:");
        attrs = d.GetType().GetCustomAttributes(true);
        foreach (Attribute attr in attrs)
        {
            Console.WriteLine(attr);
        }
    }
}

示例输出

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

请参见

参考

反射(C# 和 Visual Basic)

特性(C# 和 Visual Basic)

创建自定义特性(C# 和 Visual Basic)

使用反射访问特性(C# 和 Visual Basic)

Attribute

System.Reflection

概念

C# 编程指南

其他资源

Visual Basic 编程指南

利用特性扩展元数据