The Microsoft C++ compiler supports boxed value types directly in the language. Information about boxed value types is emitted into metadata as a custom modifier, where the modifier decorates a reference to the value type being boxed.

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:

• Using methods in the System.Reflection.Emit.TypeBuilder class such as erload:System.Reflection.Emit.TypeBuilder.DefineMethod, erload:System.Reflection.Emit.TypeBuilder.DefineField, erload:System.Reflection.Emit.TypeBuilder.DefineConstructor, and erload:System.Reflection.Emit.TypeBuilder.DefineProperty.

• Generating a Microsoft intermediate language (MSIL) instruction file that contains calls to modopt and modreq, and assembling the file with the MSIL Assembler (Ilasm.exe).

• Using the unmanaged reflection API.

Namespace: System.Runtime.CompilerServices
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 2.0.0.0, 4.0.0.0
Since: .NET 2.0