14.4.3.1 Invocations on boxed instances

A function member implemented in a value-type can be invoked through a boxed
instance of that value-type
in the following situations:
?When the function member is an override of a method inherited from type
object and is invoked
through an instance expression of type object.
?When the function member is an implementation of an interface function
member and is invoked
through an instance expression of an interface-type.
?When the function member is invoked through a delegate.
In these situations, the boxed instance is considered to contain a variable
of the value-type, and this variable
becomes the variable referenced by this within the function member
invocation. [Note: In particular, this
means that when a function member is invoked on a boxed instance, it is
possible for the function member to
modify the value contained in the boxed instance. end note]