C# Language Specification

Boxing conversions

A boxing conversion permits a value-type to be implicitly converted to a reference type. A boxing conversion exists from any non-nullable-value-type to object and dynamic, to System.ValueType and to any interface-type implemented by the non-nullable-value-type. Furthermore an enum-type can be converted to the type System.Enum.

A boxing conversion exists from a nullable-type to a reference type, if and only if a boxing conversion exists from the underlying non-nullable-value-type to the reference type.

A value type has a boxing conversion to an interface type I if it has a boxing conversion to an interface type I₀ and I₀ has an identity conversion to I.

A value type has a boxing conversion to an interface type I if it has a boxing conversion to an interface or delegate type I₀ and I₀ is variance-convertible (§13.1.3.2) to I.

Boxing a value of a non-nullable-value-type consists of allocating an object instance and copying the value-type value into that instance. A struct can be boxed to the type System.ValueType, since that is a base class for all structs (§11.3.2).

Boxing a value of a nullable-type proceeds as follows:

• 
  If the source value is null (HasValue property is false), the result is a null reference of the target type.
  
• 
  Otherwise, the result is a reference to a boxed T produced by unwrapping and boxing the source value.
  

Boxing conversions are described further in §4.3.1.

8.