Symbol constructor Null safety

const Symbol(
  1. String name

Constructs a new Symbol representing the provided name.

The name must be a valid public Dart member name, public constructor name, or library name, optionally qualified.

A qualified name is a valid name preceded by a public identifier name and a '.', e.g., is a qualified version of baz=. That means that the content of the name String must be either

  • a valid public Dart identifier (that is, an identifier not starting with "_"),
  • such an identifier followed by "=" (a setter name),
  • the name of a declarable operator (one of "+", "-", "*", "/", "%", "~/", "&", "|", "^", "~", "<<", ">>", "<", "<=", ">", ">=", "==", "[]", "[]=", or "unary-"),
  • any of the above preceded by any number of qualifiers, where a qualifier is a non-private identifier followed by '.',
  • or the empty string (the default name of a library with no library name declaration).

Symbol instances created from the same name are equal, but not necessarily identical, but symbols created as compile-time constants are canonicalized, as all other constant object creations.

assert(Symbol("foo") == Symbol("foo"));
assert(identical(const Symbol("foo"), const Symbol("foo")));

If name is a single identifier that does not start with an underscore, or it is a qualified identifier, or it is an operator name different from unary-, then the result of const Symbol(name) is the same instance that the symbol literal created by prefixing # to the content of name would evaluate to.

assert(Symbol("foo") == #foo);
assert(Symbol("[]=") == #[]=]);
assert(Symbol("") ==;
assert(identical(const Symbol("foo"), #foo));
assert(identical(const Symbol("[]="), #[]=));
assert(identical(const Symbol(""),;

This constructor cannot create a Symbol instance that is equal to a private symbol literal like #_foo.

const Symbol("_foo") // Invalid

The created instance overrides Object.==.

The following text is non-normative:

Creating non-const Symbol instances may result in larger output. If possible, use MirrorsUsed from "dart:mirrors" to specify which names might be passed to this constructor.


const factory Symbol(String name) = internal.Symbol;