Interface indexers are declared using interface-indexer-declarations:
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 (§10.8).
The accessors of an interface indexer declaration correspond to the accessors of a class indexer declaration (§???), except that no modifiers can be specified and the accessor body must always be a semicolon. Thus, the accessors simply indicate whether the indexer is read-write, read-only, or write-only.