How to: Load Assemblies into the Reflection-Only Context

Article
11/03/2006

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

Use the ReflectionOnlyLoad 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 method overload.

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.
If the assembly has dependencies, the ReflectionOnlyLoad method does not load them. If you need to examine them, you must load them yourself,.
Determine whether an assembly is loaded into the reflection-only context by using the assembly's ReflectionOnly property.

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.

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 System.Reflection.CustomAttributeData.GetCustomAttributes(System.Reflection.Assembly) method to obtain CustomAttributeData objects representing the attributes applied to an assembly, member, module, or parameter.

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.

Example

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
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

' 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:

using System;
using System.Reflection;
using 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
};

// An example attribute. The attribute can be applied to all
// targets, from assemblies to parameters.
//
[AttributeUsage(AttributeTargets.All)]
public class ExampleAttribute : Attribute
{
    // Data for properties.
    private ExampleKind kindValue;
    private string noteValue;

    // Constructors. The parameterless constructor (.ctor) calls
    // the constructor that specifies ExampleKind, and supplies the
    // default value.
    //
    public ExampleAttribute(ExampleKind initKind)
    {
        kindValue = initKind;
    }
    public ExampleAttribute() : this(ExampleKind.FirstKind) {}

    // Properties. The Note property must be read/write, so that it
    // can be used as a named parameter.
    //
    public ExampleKind Kind { get { return kindValue; }}
    public string Note    
    {
        get { return noteValue; }
        set { noteValue = value; }
    }
}

// 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 void TestMethod([Example] object arg) { }

    // 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 static void Main()
    {
        Assembly asm = Assembly.ReflectionOnlyLoad("Source");
        Type t = asm.GetType("Test");
        MethodInfo m = t.GetMethod("TestMethod");
        ParameterInfo[] p = m.GetParameters();

        Console.WriteLine("\r\nAttributes for assembly: {0}", asm);
        ShowAttributeData(CustomAttributeData.GetCustomAttributes(asm));
        Console.WriteLine("\r\nAttributes for type: {0}", t);
        ShowAttributeData(CustomAttributeData.GetCustomAttributes(t));
        Console.WriteLine("\r\nAttributes for member: {0}", m);
        ShowAttributeData(CustomAttributeData.GetCustomAttributes(m));
        Console.WriteLine("\r\nAttributes for parameter: {0}", p);
        ShowAttributeData(CustomAttributeData.GetCustomAttributes(p[0]));
    }

    private static void ShowAttributeData(
        IList<CustomAttributeData> attributes)
    {
        foreach( CustomAttributeData cad in attributes )
        {
            Console.WriteLine("   {0}", cad);
            Console.WriteLine("      Constructor: {0}", cad.Constructor);

            Console.WriteLine("      Constructor arguments:");
            foreach( CustomAttributeTypedArgument cata 
                in cad.ConstructorArguments )
            {
                Console.WriteLine("         Type: {0} Value: {1}", 
                    cata.ArgumentType, cata.Value);
            }

            Console.WriteLine("      Named arguments:");
            foreach( CustomAttributeNamedArgument cana 
                in cad.NamedArguments )
            {
                CustomAttributeTypedArgument cata = cana.TypedValue;
                Console.WriteLine("         MemberInfo: {0}", 
                    cana.MemberInfo);
                Console.WriteLine("         Type: {0} Value: {1}", 
                    cata.ArgumentType, cata.Value);
            }
        }
    }
}

/* 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:
*/

using namespace System;
using namespace System::Reflection;
using namespace System::Collections::Generic;

// An enumeration used by the ExampleAttribute class.
public enum class ExampleKind
{
   FirstKind, SecondKind, ThirdKind, FourthKind
};

// An example attribute. The attribute can be applied to all
// targets, from assemblies to parameters.
//
[AttributeUsage(AttributeTargets::All)]
public ref class ExampleAttribute: public Attribute
{
private:
   // Data for properties.
   ExampleKind kindValue;
   String^ noteValue;

   // Constructors. The parameterless constructor (.ctor) calls
   // the constructor that specifies ExampleKind, and supplies the
   // default value.
   //
   void ExampleAttributeInitialize( ExampleKind initKind )
   {
      kindValue = initKind;
   }
public:
   ExampleAttribute()
   {
      ExampleAttributeInitialize( ExampleKind::FirstKind );
   }
   ExampleAttribute( ExampleKind initKind )
   {
      ExampleAttributeInitialize( initKind );
   }

   // Properties. The Note property must be read/write, so that it
   // can be used as a named parameter.
   //
   property ExampleKind Kind 
   {
      ExampleKind get()
      {
         return kindValue;
      }
   }
   property String^ Note 
   {
      String^ get()
      {
         return noteValue;
      }

      void set( String^ value )
      {
         noteValue = value;
      }
   }
};

// The example attribute is applied to the assembly.
[assembly:Example(ExampleKind::ThirdKind,Note="This is a note on the assembly.")];

// The example attribute is applied to the test class.
//
[Example(ExampleKind::SecondKind,Note="This is a note on the class.")]
public ref class Test
{
public:
   // 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.")]
   void TestMethod( [Example] Object^ arg ){}

   // 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.
   //
   static void Main()
   {
      Assembly^ assembly = Assembly::ReflectionOnlyLoad( "Source" );
      Type^ t = assembly->GetType( "Test" );
      MethodInfo^ m = t->GetMethod( "TestMethod" );
      array<ParameterInfo^>^p = m->GetParameters();

      Console::WriteLine( "\r\nAttributes for assembly: {0}", assembly );
      ShowAttributeData( CustomAttributeData::GetCustomAttributes( assembly ) );
      Console::WriteLine( "\r\nAttributes for type: {0}", t );
      ShowAttributeData( CustomAttributeData::GetCustomAttributes( t ) );
      Console::WriteLine( "\r\nAttributes for member: {0}", m );
      ShowAttributeData( CustomAttributeData::GetCustomAttributes( m ) );
      Console::WriteLine( "\r\nAttributes for parameter: {0}", p );
      ShowAttributeData( CustomAttributeData::GetCustomAttributes( p[ 0 ] ) );
   }

private:
   static void ShowAttributeData( IList< CustomAttributeData^ >^ attributes )
   {
      for each ( CustomAttributeData^ cad in attributes )
      {
         Console::WriteLine( "   {0}", cad );
         Console::WriteLine( "      Constructor: {0}", cad->Constructor );

         Console::WriteLine( "      Constructor arguments:" );
         for each ( CustomAttributeTypedArgument^ cata in cad->ConstructorArguments )
         {
            Console::WriteLine( "         Type: {0} Value: {1}",
               cata->ArgumentType, cata->Value );
         }

         Console::WriteLine( "      Named arguments:" );
         for each ( CustomAttributeNamedArgument cana in cad->NamedArguments )
         {
            CustomAttributeTypedArgument cata = cana.TypedValue;
            Console::WriteLine( "         MemberInfo: {0}", cana.MemberInfo );
            Console::WriteLine( "         Type: {0} Value: {1}", cata.ArgumentType, cata.Value );
         }
      }
   }
};

int main()
{
   Test::Main();
}

/* 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:
*/

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How to: Load Assemblies into the Reflection-Only Context

To load an assembly into the reflection-only load context

Example

See Also

Reference

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