Boxing is wrapping a value type variable such as Windows::Foundation::DateTime—or a fundamental scalar type such as int—in a ref class when the variable is passed to a method that takes Platform::Object^ as its input type.

Although you don't have to explicitly box a variable to pass it to a method parameter of type Platform::Object^, you do have to explicitly cast back to the original type when you retrieve values that have been previously boxed.

[!code-cppcx_boxing#01]

C# and Visual Basic support the concept of nullable value types. In C++/CX, you can use the Platform::IBox<T> type to expose public methods that support nullable value type parameters. The following example shows a C++/CX public method that returns null when a C# caller passes null for one of the arguments.

[!code-cppcx_boxing#02]

In a C# XAML client, you can consume it like this:

  
// C# client code  
    BoxingDemo.Class1 obj = new BoxingDemo.Class1();  
    int? a = null;  
    int? b = 5;  
    var result = obj.Multiply(a,b); //result = null  
  

Type System (C++/CX)
Casting (C++/CX)
Visual C++ Language Reference
Namespaces Reference