struct Foo: Identifiable { typealias ID = Int var id: ID var value: String } let foos: [Foo] = [ .init(id: 1, value: "ABC"), .init(id: 2, value: "XYZ"), ] var idToFoo: [Foo.ID: Foo] = .init(uniqueKeysWithValues: foos.map { ($0.id, $0) }) print(idToFoo) idToFoo[1]?.id = 999 print(idToFoo)