Interface indexers are declared using interface-indexer-declarations: interface-indexer-declaration : attributesnewtypethis[formal-parameter-list]{interface-accessors} The attributes, type, and formal-parameter-list of an interface indexer declaration have the same meaning as those of an indexer declaration in a class (17.8). The accessors of an interface indexer declaration correspond to the accessors of a class indexer declaration (17.8), except that the accessor body must always be a semicolon. Thus, the accessors simply indicate whether the indexer is read-write, read-only, or write-only.