CustomAttributeData Class
Assembly: mscorlib (in mscorlib.dll)
'Declaration <SerializableAttribute> _ <ComVisibleAttribute(True)> _ Public NotInheritable Class CustomAttributeData 'Usage Dim instance As CustomAttributeData
/** @attribute SerializableAttribute() */ /** @attribute ComVisibleAttribute(true) */ public final class CustomAttributeData
SerializableAttribute ComVisibleAttribute(true) public final class CustomAttributeData
Not applicable.
Code that is being examined in the reflection-only context cannot be executed, so it is not always possible to examine custom attributes by creating instances of them and then examining their properties, using methods like System.Attribute.GetCustomAttributes, System.Reflection.MemberInfo.GetCustomAttributes, and so on. If the code for the attribute type itself is loaded into the reflection-only context, it cannot be executed.
The CustomAttributeData class allows examination of custom attributes in the reflection-only context by providing an abstraction for attributes. The members of this class can be used to obtain values for the positional arguments and named arguments of the attribute. For enumerations, the value is provided using the underlying type of the enumeration. The name of the enumeration is supplied along with the value, in the CustomAttributeTypedArgument structure.
CustomAttributeData can be used in the execution context as well as in the reflection-only context. For example, you might want to avoid loading the assembly that contains the code for a custom attribute. Using the CustomAttributeData class is different from using methods like System.Attribute.GetCustomAttributes:
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The properties and methods of CustomAttributeData only provide you with the values that were specified for the attribute instance, not the semantics of the constructor. For example, a string argument of an attribute might be converted internally to some other representation, and returned in a canonical form; or a property might have side effects when the actual attribute code is executed.
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The properties and methods of CustomAttributeData do not allow you to retrieve the custom attributes inherited from base classes.
To create instances of the CustomAttributeData class, use the static (Shared in Visual Basic) GetCustomAttributes factory methods.
The following example defines a custom attribute with four constructors and four properties. Two of the properties are read-only, and are set by using the positional parameters of the constructors. The other two properties are read/write, and can be set only by using named arguments. One positional property is an array of strings, and one named property is an array of integers.
The attribute is applied to the assembly, to a type declared in the assembly, to a method of the type, and to a parameter of the method. Different constructors are used for these cases. When executed, the assembly loads itself into the reflection-only context and displays information about the custom attributes that were applied to it and to the type and members it contains.
The attribute that is applied to the type demonstrates array properties, with both positional and named arguments.
Imports System Imports System.Reflection Imports System.Collections.Generic ' The example attribute is applied to the assembly. <Assembly:Example(ExampleKind.ThirdKind, Note:="This is a note on the assembly.")> ' An enumeration used by the ExampleAttribute class. Public Enum ExampleKind FirstKind SecondKind ThirdKind FourthKind End Enum ' An example attribute. The attribute can be applied to all ' targets, from assemblies to parameters. ' <AttributeUsage(AttributeTargets.All)> _ Public Class ExampleAttribute Inherits Attribute ' Data for properties. Private kindValue As ExampleKind Private noteValue As String ' Constructors. The parameterless constructor (.ctor) calls ' the constructor that specifies ExampleKind, and supplies the ' default value. ' Public Sub New(ByVal initKind As ExampleKind) kindValue = initKind End Sub Public Sub New() Me.New(ExampleKind.FirstKind) End Sub ' Properties. The Note property must be read/write, so that it can ' be used as a named parameter. ' Public ReadOnly Property Kind As ExampleKind Get Return kindValue End Get End Property Public Property Note As String Get Return noteValue End Get Set noteValue = value End Set End Property End Class ' The example attribute is applied to the test class. ' <Example(ExampleKind.SecondKind, Note:="This is a note on the class.")> _ Public Class Test ' The example attribute is applied to a method, using the ' parameterless constructor and supplying a named argument. ' The attribute is also applied to the method parameter. ' <Example(Note:="This is a note on a method.")> _ Public Sub TestMethod(<Example()> ByVal arg As Object) End Sub ' Sub Main gets objects representing the assembly, the test ' type, the test method, and the method parameter. Custom ' attribute data is displayed for each of these. ' Public Shared Sub Main() Dim asm As [Assembly] = Assembly.ReflectionOnlyLoad("Source") Dim t As Type = asm.GetType("Test") Dim m As MethodInfo = t.GetMethod("TestMethod") Dim p() As ParameterInfo = m.GetParameters() Console.WriteLine(vbCrLf & "Attributes for assembly: {0}", asm) ShowAttributeData(CustomAttributeData.GetCustomAttributes(asm)) Console.WriteLine(vbCrLf & "Attributes for type: {0}", t) ShowAttributeData(CustomAttributeData.GetCustomAttributes(t)) Console.WriteLine(vbCrLf & "Attributes for member: {0}", m) ShowAttributeData(CustomAttributeData.GetCustomAttributes(m)) Console.WriteLine(vbCrLf & "Attributes for parameter: {0}", p) ShowAttributeData(CustomAttributeData.GetCustomAttributes(p(0))) End Sub Private Shared Sub ShowAttributeData( _ ByVal attributes As IList(Of CustomAttributeData)) For Each cad As CustomAttributeData _ In CType(attributes, IEnumerable(Of CustomAttributeData)) Console.WriteLine(" {0}", cad) Console.WriteLine(" Constructor: {0}", cad.Constructor) Console.WriteLine(" Constructor arguments:") For Each cata As CustomAttributeTypedArgument _ In CType(cad.ConstructorArguments, IEnumerable(Of CustomAttributeTypedArgument)) Console.WriteLine(" Type: {0} Value: {1}", _ cata.ArgumentType, cata.Value) Next Console.WriteLine(" Named arguments:") For Each cana As CustomAttributeNamedArgument _ In CType(cad.NamedArguments, IEnumerable(Of CustomAttributeNamedArgument)) Dim cata As CustomAttributeTypedArgument = _ cana.TypedValue Console.WriteLine(" MemberInfo: {0}", _ cana.MemberInfo) Console.WriteLine(" Type: {0} Value: {1}", _ cata.ArgumentType, cata.Value) Next Next End Sub End Class ' This code example produces output similar to the following: ' 'Attributes for assembly: source, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null ' [ExampleAttribute((ExampleKind)2, Note = "This is a note on the assembly.")] ' Constructor: Void .ctor(ExampleKind) ' Constructor arguments: ' Type: ExampleKind Value: 2 ' Named arguments: ' MemberInfo: System.String Note ' Type: System.String Value: This is a note on the assembly. ' [System.Runtime.CompilerServices.CompilationRelaxationsAttribute((Int32)8)] ' Constructor: Void .ctor(Int32) ' Constructor arguments: ' Type: System.Int32 Value: 8 ' Named arguments: ' 'Attributes for type: Test ' [ExampleAttribute((ExampleKind)1, Note = "This is a note on the class.")] ' Constructor: Void .ctor(ExampleKind) ' Constructor arguments: ' Type: ExampleKind Value: 1 ' Named arguments: ' MemberInfo: System.String Note ' Type: System.String Value: This is a note on the class. ' 'Attributes for member: Void TestMethod(System.Object) ' [ExampleAttribute(Note = "This is a note on a method.")] ' Constructor: Void .ctor() ' Constructor arguments: ' Named arguments: ' MemberInfo: System.String Note ' Type: System.String Value: This is a note on a method. ' 'Attributes for parameter: System.Object arg ' [ExampleAttribute()] ' Constructor: Void .ctor() ' Constructor arguments: ' Named arguments:
Windows 98, Windows Server 2000 SP4, Windows Millennium Edition, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition
The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.