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4.3 Boxing and Unboxing

The concept of boxing and unboxing is central to C#’s type system. It provides a bridge between value-types and reference-types by permitting any value of a value-type to be converted to and from type object. Boxing and unboxing enables a unified view of the type system wherein a value of any type can ultimately be treated as an object.

4.3.1 Boxing Conversions

A boxing conversion permits a value-type to be implicitly converted to a reference-type. The following boxing conversions exist:

  • From any value-type to the type object.
  • From any value-type to the type System.ValueType.
  • From any non-nullable-value-type to any interface-type implemented by the value-type.
  • From any nullable-type to any interface-type implemented by the underlying type of the nullable-type.
  • From any enum-type to the type System.Enum.
  • From any nullable-type with an underlying enum-type to the type System.Enum.

Note that an implicit conversion from a type parameter will be executed as a boxing conversion if at runtime it ends up converting from a value type to a reference type (§6.1.10).

Boxing a value of a non-nullable-value-type consists of allocating an object instance and copying the non-nullable-value-type value into that instance.

Boxing a value of a nullable-type produces a null reference if it is the null value (HasValue is false), or the result of unwrapping and boxing the underlying value otherwise.

The actual process of boxing a value of a non-nullable-value-type is best explained by imagining the existence of a generic boxing class, which behaves as if it were declared as follows:

sealed class Box<T>: System.ValueType
    T value;

    public Box(T t) {
        value = t;

Boxing of a value v of type T now consists of executing the expression new Box<T>(v) and returning the resulting instance as a value of type object. Thus the statements

int i = 123;
object box = i;

conceptually correspond to

int i = 123;
object box = new Box<int>(i);

A boxing class like Box<T> above doesn’t actually exist, and the dynamic type of a boxed value isn’t actually a class type. Instead, a boxed value of type T has the dynamic type T, and a dynamic type check using the is operator can simply reference type T. For example,

int i = 123;
object box = i;
if (box is int) {
    Console.Write("Box contains an int");

will output the string “Box contains an int” on the console.

A boxing conversion implies making a copy of the value being boxed. This is different from a conversion of a reference-type to type object, in which the value continues to reference the same instance and simply is regarded as the less derived type object. For example, given the declaration

struct Point
    public int x, y;
    public Point(int x, int y) {

        this.x = x;
        this.y = y;

the following statements

Point p = new Point(10, 10);
object box = p;
p.x = 20;

will output the value 10 on the console because the implicit boxing operation that occurs in the assignment of p to box causes the value of p to be copied. Had Point been declared a class instead, the value 20 would be output because p and box would reference the same instance.

4.3.2 Unboxing Conversions

An unboxing conversion permits a reference-type to be explicitly converted to a value-type. The following unboxing conversions exist:

  • From the type object to any value-type.
  • From the type System.ValueType to any value-type.
  • From any interface-type to any non-nullable-value-type that implements the interface-type.
  • From any interface-type to any nullable-type whose underlying type implements the interface-type.
  • From the type System.Enum to any enum-type.
  • From the type System.Enum to any nullable-type with an underlying enum-type.

Note that an explicit conversion to a type parameter will be executed as an unboxing conversion if at runtime it ends up converting from a reference type to a value type (§6.2.6).

An unboxing operation to a non-nullable-value-type consists of first checking that the object instance is a boxed value of the given non-nullable-value-type, and then copying the value out of the instance.

Unboxing to a nullable-type produces the null value of the nullable-type if the source operand is null, or the wrapped result of unboxing the object instance to the underlying type of the nullable-type otherwise.

Referring to the imaginary boxing class described in the previous section, an unboxing conversion of an object box to a value-type T consists of executing the expression ((Box<T>)box).value. Thus the statements

object box = 123;
int i = (int)box;

conceptually correspond to

object box = new Box<int>(123);
int i = ((Box<int>)box).value;

For an unboxing conversion to a given non-nullable-value-type to succeed at runtime, the value of the source operand must be a reference to a boxed value of that non-nullable-value-type. If the source operand is null, a System.NullReferenceException is thrown. If the source operand is a reference to an incompatible object, a System.InvalidCastException is thrown.

For an unboxing conversion to a given nullable-type to succeed at runtime, the value of the source operand must be either null or a reference to a boxed value of the underlying non-nullable-value-type of the nullable-type. If the source operand is a reference to an incompatible object, a System.InvalidCastException is thrown.

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