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 .NET. Code loaded into this context can only be examined; it can't be executed. This means that objects can't be created, because constructors can't be executed. Because the code can't 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

   You can't 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

   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.

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

// 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;
 array<String^>^ arrayStrings;
 array<int>^ arrayNumbers;

 // Constructors. 
 void ExampleAttributeInitialize( ExampleKind initKind, array<String^>^ initStrings )
 {
 kindValue = initKind;
 arrayStrings = initStrings;
 }
public:
 ExampleAttribute()
 {
 ExampleAttributeInitialize( ExampleKind::FirstKind, nullptr );
 }
 ExampleAttribute( ExampleKind initKind )
 {
 ExampleAttributeInitialize( initKind, nullptr );
 }
 ExampleAttribute( ExampleKind initKind, array<String^>^ initStrings )
 {
 ExampleAttributeInitialize( initKind, initStrings );
 }

 // Properties. The Note and Numbers properties must be read/write, so they
 // can be used as named parameters.
 //
 property ExampleKind Kind 
 {
 ExampleKind get()
 {
 return kindValue;
 }
 }
 property array<String^>^ Strings
 {
 array<String^>^ get()
 {
 return arrayStrings;
 }
 }
 property String^ Note 
 {
 String^ get()
 {
 return noteValue;
 }

 void set( String^ value )
 {
 noteValue = value;
 }
 }
 property array<int>^ Numbers
 {
 array<int>^ get()
 {
 return arrayNumbers;
 }

 void set( array<int>^ value )
 {
 arrayNumbers = 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, 
 gcnew array<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 = gcnew array<int> { 53, 57, 59 })] 
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 ShowValueOrArray(CustomAttributeTypedArgument^ cata)
 {
 if (cata->Value->GetType() == ReadOnlyCollection<CustomAttributeTypedArgument>::typeid)
 {
 Console::WriteLine(" Array of '{0}':", cata->ArgumentType);

 for each (CustomAttributeTypedArgument^ cataElement in 
 (ReadOnlyCollection<CustomAttributeTypedArgument>^) cata->Value)
 {
 Console::WriteLine(" Type: '{0}' Value: '{1}'",
 cataElement->ArgumentType, cataElement->Value);
 }
 }
 else
 {
 Console::WriteLine( " Type: '{0}' Value: '{1}'",
 cata->ArgumentType, cata->Value );
 }
 }

 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 )
 {
 ShowValueOrArray(cata);
 }

 Console::WriteLine( " Named arguments:" );
 for each ( CustomAttributeNamedArgument cana in cad->NamedArguments )
 {
 Console::WriteLine( " MemberInfo: '{0}'", cana.MemberInfo );
 ShowValueOrArray(cana.TypedValue);
 }
 }
 }
};

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'
 [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:
*/

using System;
using System.Reflection;
using System.Collections.Generic;
using 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
};

// 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;
 private string[] arrayStrings;
 private int[] arrayNumbers;

 // Constructors. The parameterless constructor (.ctor) calls
 // the constructor that specifies ExampleKind and an array of
 // strings, and supplies the default values.
 //
 public ExampleAttribute(ExampleKind initKind, string[] initStrings)
 {
 kindValue = initKind;
 arrayStrings = initStrings;
 }
 public ExampleAttribute(ExampleKind initKind) : this(initKind, null) {}
 public ExampleAttribute() : this(ExampleKind.FirstKind, null) {}

 // Properties. The Note and Numbers properties must be read/write, so they
 // can be used as named parameters.
 //
 public ExampleKind Kind { get { return kindValue; }}
 public string[] Strings { get { return arrayStrings; }}
 public string Note
 {
 get { return noteValue; }
 set { noteValue = value; }
 }
 public int[] Numbers
 {
 get { return arrayNumbers; }
 set { arrayNumbers = value; }
 }
}

// 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 int[] { 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 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($"\r\nAttributes for assembly: '{asm}'");
 ShowAttributeData(CustomAttributeData.GetCustomAttributes(asm));
 Console.WriteLine($"\r\nAttributes for type: '{t}'");
 ShowAttributeData(CustomAttributeData.GetCustomAttributes(t));
 Console.WriteLine($"\r\nAttributes for member: '{m}'");
 ShowAttributeData(CustomAttributeData.GetCustomAttributes(m));
 Console.WriteLine($"\r\nAttributes for parameter: '{p}'");
 ShowAttributeData(CustomAttributeData.GetCustomAttributes(p[0]));
 }

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

 Console.WriteLine(" Constructor arguments:");
 foreach( CustomAttributeTypedArgument cata
 in cad.ConstructorArguments )
 {
 ShowValueOrArray(cata);
 }

 Console.WriteLine(" Named arguments:");
 foreach( CustomAttributeNamedArgument cana
 in cad.NamedArguments )
 {
 Console.WriteLine($" MemberInfo: '{cana.MemberInfo}'");
 ShowValueOrArray(cana.TypedValue);
 }
 }
 }

 private static void ShowValueOrArray(CustomAttributeTypedArgument cata)
 {
 if (cata.Value.GetType() == typeof(ReadOnlyCollection<CustomAttributeTypedArgument>))
 {
 Console.WriteLine($" Array of '{cata.ArgumentType}':");

 foreach (CustomAttributeTypedArgument cataElement in
 (ReadOnlyCollection<CustomAttributeTypedArgument>) cata.Value)
 {
 Console.WriteLine($" Type: '{cataElement.ArgumentType}' Value: '{cataElement.Value}'");
 }
 }
 else
 {
 Console.WriteLine($" Type: '{cata.ArgumentType}' Value: '{cata.Value}'");
 }
 }
}

/* 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:
*/

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:

See also

• ReflectionOnlyLoad
• ReflectionOnly
• CustomAttributeData