Programming Visual Basic 2005

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.

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 18-5.

Figure 18-5. Boxing reference types

Boxing is implicit when you provide a value type where a reference is expected and the value is implicitly boxed. You can, of course, explicitly cast the value type to a reference type:

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

This is not necessary, however, since the compiler will box the value for you:

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

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 18-6 illustrates unboxing.

Figure 18-6. Unboxing