Interface indexers are declared using interface-indexer-declarations:

interface-indexer-declaration

attributes_optnew _opt typethis [ 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.