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How to: Load Assemblies into the Reflection-Only Context

The reflection-only load context allows you to examine assemblies compiled for other platforms or for other versions of the .NET Framework. Code loaded into this context can only be examined; it cannot be executed. This means that objects cannot be created, because constructors cannot be executed. Because the code cannot be executed, dependencies are not automatically loaded. If you need to examine them, you must load them yourself.

To load an assembly into the reflection-only load context

  1. Use the ReflectionOnlyLoad(String) method overload to load the assembly given its display name, or the ReflectionOnlyLoadFrom method to load the assembly given its path. If the assembly is a binary image, use the ReflectionOnlyLoad(Byte()) method overload.

    Note Note

    You cannot use the reflection-only context to load a version of mscorlib.dll from a version of the .NET Framework other than the version in the execution context.

  2. If the assembly has dependencies, the ReflectionOnlyLoad method does not load them. If you need to examine them, you must load them yourself,.

  3. Determine whether an assembly is loaded into the reflection-only context by using the assembly's ReflectionOnly property.

  4. If attributes have been applied to the assembly or to types in the assembly, examine those attributes by using the CustomAttributeData class to ensure that no attempt is made to execute code in the reflection-only context. Use the appropriate overload of the CustomAttributeData.GetCustomAttributes method to obtain CustomAttributeData objects representing the attributes applied to an assembly, member, module, or parameter.

    Note Note

    Attributes applied to the assembly or to its contents might be defined in the assembly, or they might be defined in another assembly loaded into the reflection-only context. There is no way to tell in advance where the attributes are defined.

The following code example shows how to examine the attributes applied to an assembly loaded into the reflection-only context.

The code example defines a custom attribute with two constructors and one property. 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. 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 types and members it contains.

Note Note

To simplify the code example, the assembly loads and examines itself. Normally, you would not expect to find the same assembly loaded into both the execution context and the reflection-only context.

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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