C# Language Specification

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:

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  When the function member is an override of a method inherited from type object and is invoked through an instance expression of type object.
  
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  When the function member is an implementation of an interface function member and is invoked through an instance expression of an interface-type.
  
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  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. 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.

6.