The only places that . appears in the grammar are:
real-literal:
decimal-digits . decimal-digits ...
. decimal-digits ...
namespace-or-type-name:
namespace-or-type-name . identifier ...
member-access:
primary-expression . identifier ...
predefined-type . identifier ...
qualified-alias-member . identifier ...
base-access:
base . identifier
unbound-type-name:
unbound-type-name . identifier
qualified-identifier:
qualified-identifier . identifier
member-name:
interface-type . identifier
indexer-declarator:
type interface-type . this
(The … means I have elided the remainder of the production rule.) In none of these cases is a .( valid as . is either followed by digits, a valid identifier, or the keyword this.