It is more subtle and powerful than that. By placing the contract at the use of a type with an interface rather than at the declaration of a type you effective

It is more subtle and powerful than that. By placing the contract at the use of a type with an interface rather than at the declaration of a type you effectively introduce a form of duck typing to a static language. You can write an interface for a type long after the type has been declared (or been declared by someone else). Interfaces can be defined and used without having to go back and change existing code. This is very lightweight.