Boxing and Unboxing (C# Programming Guide) 

Boxing and unboxing enable value types to be treated as objects. Boxing a value type packages it inside an instance of the Object reference type. This allows the value type to be stored on the garbage collected heap. Unboxing extracts the value type from the object. In this example, the integer variable i is boxed and assigned to object o.

int i = 123;
object o = (object) i;  // boxing

The object o can then be unboxed and assigned to integer variable i:

o = 123;
i = (int) o;  // unboxing

Performance

In relation to simple assignments, boxing and unboxing are computationally expensive processes. When a value type is boxed, an entirely new object must be allocated and constructed. To a lesser degree, the cast required for unboxing is also expensive computationally. For more information, see Performance.

Related Sections

For more information:

• Boxing Conversion

• Unboxing Conversion

• Reference Types

• Value Types

C# Language Specification

For more information, see the following section in the C# Language Specification:

• 4.3 Boxing and Unboxing

See Also

Concepts

C# Programming Guide
Boxed Value Types