IsByValue Class
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IsByValue Class

Indicates that a modified method argument should be interpreted as having object passed-by-value semantics. This modifier is applied to reference types. This class cannot be inherited.

Namespace:  System.Runtime.CompilerServices
Assembly:  mscorlib (in mscorlib.dll)

public static class IsByValue

The IsByValue class is used by the Microsoft C++ compiler to denote method parameters and return values whose semantics follow the C++ rules for objects passed by value.

Compilers emit custom modifiers within metadata to change the way that the just-in-time (JIT) compiler handles values when the default behavior is not appropriate. When the JIT compiler encounters a custom modifier, it handles the value in the way that the modifier specifies. Compilers can apply custom modifiers to methods, parameters, and return values. The JIT compiler must respond to required modifiers but can ignore optional modifiers.

You can emit custom modifiers into metadata using one of the following techniques:

The following example demonstrates how to emit an IsByValue object into an assembly using reflection.

#using <mscorlib.dll>

using namespace System;
using namespace System::Reflection;
using namespace System::Reflection::Emit;
using namespace System::Runtime::CompilerServices;
using namespace System::Threading;

ref class CodeEmitter
    AssemblyBuilder^ asmBuilder;
    String^ asmName;
    ModuleBuilder^ modBuilder;

    void prepareAssembly(String^ name){

        // Check the input. 

            throw gcnew ArgumentNullException("AssemblyName");

        asmName = name;

        // Create an AssemblyName object and set the name.
        AssemblyName^ asmName = gcnew AssemblyName();

        asmName->Name = name;

        // Use the AppDomain class to create an AssemblyBuilder instance.

        AppDomain^ currentDomain = Thread::GetDomain();

        asmBuilder = currentDomain->DefineDynamicAssembly(asmName,AssemblyBuilderAccess::RunAndSave);

        // Create a dynamic module.
        modBuilder = asmBuilder->DefineDynamicModule(name);


    // Constructor.
    CodeEmitter(String ^ AssemblyName){


    // Create a new type.
    TypeBuilder^ CreateType(String^ name){

        // Check the input. 

            throw gcnew ArgumentNullException("AssemblyName");

        return modBuilder->DefineType( name );

    // Write the assembly. 
    void WriteAssembly(MethodBuilder^ entryPoint){

        // Check the input. 

            throw gcnew ArgumentNullException("entryPoint");

        asmBuilder->SetEntryPoint( entryPoint );
        asmBuilder->Save( asmName );


void main()

    // Create a CodeEmitter to handle assembly creation.
    CodeEmitter ^ e = gcnew CodeEmitter("program.exe");

    // Create a new type.
    TypeBuilder^ mainClass = e->CreateType("MainClass");

    // Create a new method.
    MethodBuilder^ mBuilder = mainClass->DefineMethod("mainMethod", MethodAttributes::Static);

    // Create an ILGenerator and emit IL for  
    // a simple "Hello World." program.
    ILGenerator^ ilGen = mBuilder->GetILGenerator();

    ilGen->Emit(OpCodes::Ldstr, "Hello World");

    array<Type^>^mType = {String::typeid};

    MethodInfo^ writeMI = Console::typeid->GetMethod( "WriteLine", mType );

    ilGen->EmitCall(OpCodes::Call, writeMI, nullptr );

    ilGen->Emit( OpCodes::Ret );

    // Apply a required custom modifier 
    // to a field. 

    array<Type^>^fType = {IsByValue::typeid};

    mainClass->DefineField("modifiedInteger", Type::GetType("System.Int32"), fType, nullptr, FieldAttributes::Private);

    // Create the type.

    // Write the assembly using a reference to  
    // the entry point.

    Console::WriteLine(L"Assembly created.");


Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0

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