Section 11.7. Boxing and Unboxing Types

11.7 Boxing and Unboxing Types

Boxing and unboxing are the processes that enable value types (e.g., integers) to be treated as reference types (objects). The value is "boxed" inside an Object and subsequently "unboxed" back to a value type. It is this process that allowed us to call the ToString( ) method on the integer in Example 11-4. (You will see additional uses for boxing and unboxing in Chapter 15.)

11.7.1 Boxing Is Implicit

Boxing is an implicit conversion of a value type to the type Object. Boxing a value allocates an instance of Object and copies the value into the new object instance, as shown in Figure 11-4.

Figure 11-4. Boxing value types

Boxing is implicit when you provide a value type where a reference is expected. The compiler notices that you've provided a value type and silently boxes it within an object. You can, of course, explicitly cast the value type to a reference type, as in the following:

 Dim myIntegerValue As Integer = 5 Dim myObject as Object = myIntegerValue ' explicitly cast to object myObject.ToString( ) 

This is not necessary, however, as the compiler will box the value for you, silently and with no action on your part:

 Dim myIntegerValue As Integer = 5 myIntegerValue.ToString( ) ' boxed for you 

11.7.2 Unboxing Must Be Explicit

To return the boxed object back to a value type, you must explicitly unbox it if Option Strict is On (as it should be). You will typically unbox by using the DirectCast( ) function or the CType( ) function. Figure 11-5 illustrates unboxing.

Figure 11-5. Unboxing

Boxing and unboxing are illustrated in Example 11-5.

Example 11-5. Boxing and unboxing

 Option Strict On Imports System Public Class UnboxingTest     Public Shared Sub Main( )         Dim myIntegerVariable As Integer = 123         ' Boxing         Dim myObjectVariable As Object = myIntegerVariable         Console.WriteLine("myObjectVariable: {0}", _               myObjectVariable.ToString( ))         ' unboxing (must be explicit)         Dim anotherIntegerVariable As Integer = _               DirectCast(myObjectVariable, Integer)         Console.WriteLine("anotherIntegerVariable: {0}", _              anotherIntegerVariable)     End Sub End Class 

  Output:  myObjectVariable: 123 anotherIntegerVariable: 123 

Example 11-5 creates an integer myIntegerVariable and implicitly boxes it when it is assigned to the object myObjectVariable; then, to exercise the newly boxed object, its value is displayed by calling toString( ).

The object is then explicitly unboxed and assigned to a new integer variable, anotherIntegerVariable, whose value is displayed to show that the value has been preserved.

Typically, you will wrap an unbox operation in a Try block, as explained in Chapter 16. If the object being unboxed is null or is a reference to an object of a different type, an InvalidCastException error occurs.

As an alternative, you can use the TypeOf( ) function, as follows :

 ' unboxing (must be explicit) If TypeOf (myObjectVariable) Is Integer Then    Dim anotherIntegerVariable As Integer = _        DirectCast(myObjectVariable, Integer)    Console.WriteLine("anotherIntegerVariable: {0}", _        anotherIntegerVariable) End If