CustomAttributeTypedArgument Structure
Represents an argument of a custom attribute in the reflection-only context, or an element of an array argument.
Assembly: mscorlib (in mscorlib.dll)
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 Attribute.GetCustomAttributes, 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 CustomAttributeNamedArgument structure is used by the CustomAttributeData class to provide access to the type and value of a positional argument specified for a custom attribute instance, without executing the attribute constructor. It also provides access to the type and value of a named argument without executing the code of the corresponding property of the custom attribute type.
The types and values of all the positional and named arguments of an attribute instance are provided by CustomAttributeTypedArgument structures. The positional attributes returned by the CustomAttributeData.ConstructorArguments property are directly represented by CustomAttributeTypedArgument structures, but the named arguments returned by the CustomAttributeData.NamedArguments property are represented by CustomAttributeNamedArgument structures; to get the CustomAttributeTypedArgument structure for a named argument, use the CustomAttributeNamedArgument.TypedValue property.
If an argument is an array of values, the Value property of the CustomAttributeTypedArgument that represents the argument returns a generic ReadOnlyCollection(Of T) of CustomAttributeTypedArgument objects. Each CustomAttributeTypedArgument object in the collection represents the corresponding element of the array.
To create instances of the CustomAttributeData class, use the static GetCustomAttributes factory method.
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 the custom attributes.
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 Imports System.Collections.ObjectModel ' 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 Private arrayStrings() As String Private arrayNumbers() As Integer ' Constructors. The parameterless constructor (.ctor) calls ' the constructor that specifies ExampleKind and an array of ' strings, and supplies the default values. ' Public Sub New(ByVal initKind As ExampleKind, ByVal initStrings() As String) kindValue = initKind arrayStrings = initStrings End Sub Public Sub New(ByVal initKind As ExampleKind) Me.New(initKind, Nothing) End Sub Public Sub New() Me.New(ExampleKind.FirstKind, Nothing) End Sub ' Properties. The Note and Numbers properties must be read/write, so they ' can be used as named parameters. ' Public ReadOnly Property Kind As ExampleKind Get Return kindValue End Get End Property Public ReadOnly Property Strings As String() Get Return arrayStrings End Get End Property Public Property Note As String Get Return noteValue End Get Set noteValue = value End Set End Property Public Property Numbers As Integer() Get Return arrayNumbers End Get Set arrayNumbers = value End Set End Property End Class ' The example attribute is applied to the test class. ' <Example(ExampleKind.SecondKind, _ New String() { "String array argument, line 1", _ "String array argument, line 2", _ "String array argument, line 3" }, _ Note := "This is a note on the class.", _ Numbers := New Integer() { 53, 57, 59 })> _ 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)) ShowValueOrArray(cata) Next Console.WriteLine(" Named arguments:") For Each cana As CustomAttributeNamedArgument _ In CType(cad.NamedArguments, IEnumerable(Of CustomAttributeNamedArgument)) Console.WriteLine(" MemberInfo: '{0}'", _ cana.MemberInfo) ShowValueOrArray(cana.TypedValue) Next Next End Sub Private Shared Sub ShowValueOrArray(ByVal cata As CustomAttributeTypedArgument) If cata.Value.GetType() Is GetType(ReadOnlyCollection(Of CustomAttributeTypedArgument)) Then Console.WriteLine(" Array of '{0}':", cata.ArgumentType) For Each cataElement As CustomAttributeTypedArgument In cata.Value Console.WriteLine(" Type: '{0}' Value: '{1}'", _ cataElement.ArgumentType, cataElement.Value) Next Else Console.WriteLine(" Type: '{0}' Value: '{1}'", _ cata.ArgumentType, cata.Value) End If 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' ' [System.Runtime.CompilerServices.CompilationRelaxationsAttribute((Int32)8)] ' Constructor: 'Void .ctor(Int32)' ' Constructor arguments: ' Type: 'System.Int32' Value: '8' ' Named arguments: ' [System.Runtime.CompilerServices.RuntimeCompatibilityAttribute(WrapNonExceptionThrows = True)] ' Constructor: 'Void .ctor()' ' Constructor arguments: ' Named arguments: ' MemberInfo: 'Boolean WrapNonExceptionThrows' ' Type: 'System.Boolean' Value: 'True' ' [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.' ' 'Attributes for type: 'Test' ' [ExampleAttribute((ExampleKind)1, new String[3] { "String array argument, line 1", "String array argument, line 2", "String array argument, line 3" }, Note = "This is a note on the class.", Numbers = new Int32[3] { 53, 57, 59 })] ' Constructor: 'Void .ctor(ExampleKind, System.String[])' ' Constructor arguments: ' Type: 'ExampleKind' Value: '1' ' Array of 'System.String[]': ' Type: 'System.String' Value: 'String array argument, line 1' ' Type: 'System.String' Value: 'String array argument, line 2' ' Type: 'System.String' Value: 'String array argument, line 3' ' Named arguments: ' MemberInfo: 'System.String Note' ' Type: 'System.String' Value: 'This is a note on the class.' ' MemberInfo: 'Int32[] Numbers' ' Array of 'System.Int32[]': ' Type: 'System.Int32' Value: '53' ' Type: 'System.Int32' Value: '57' ' Type: 'System.Int32' Value: '59' ' '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:
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