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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteCan you use constructor tear-offs with Dart enhanced enums? Not with a generative enum constructor: Dart only permits it to initialize declared enum values or redirect within the same enum. Use a factory constructor when you need a function-like enum API; it can return an existing value, but it cannot create a new one.
What constructor tear-offs do—and why enums differ
A constructor tear-off is a constructor reference used as a function value. For an ordinary class, you can pass StringBuffer.new where a compatible callback is expected instead of writing a forwarding lambda such as (text) => StringBuffer(text). As the Dart constructors guide puts it, “Lambdas serve as a wrapper for the constructor, whereas a tear-off is the constructor.” Dart constructors guide.
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An enhanced enum can have fields, methods and constructors, but its instances are the finite set of values declared in the enum. A generative constructor initializes those declared values; it is not a general-purpose way to create more enum objects. Consequently, MyEnum.new is not a valid callback reference to a generative enum constructor.
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Why the analyzer rejects a generative enum constructor reference
The restriction is explicit in Dart’s analyzer diagnostic: “Generative enum constructors can’t be torn off.” A generative enum constructor may be used to initialize one of that enum’s declared values or for a same-enum redirect, not as an independent function reference. See the analyzer diagnostic and the enhanced-enum specification.
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For example, in this conventional enhanced-enum declaration, const Mode(1) initializes a declared value; it does not expose Mode.new as a callback:
enum Mode {
compact(1),
expanded(2);
final int scale;
const Mode(this.scale);
}Enhanced enums require language version 2.17 or later. Their instance variables must be final, and generative constructors must be constant. The Dart enum documentation describes those constraints.
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Use a factory when a callback should select an existing enum value
A factory constructor can provide constructor-shaped invocation syntax while returning one of the enum’s already declared values. The analyzer documentation gives this minimal example:
enum E {
a(0);
const E(int x);
factory E.c(int x) => a;
}E.c is a factory path, not a tear-off of the generative constructor. It returns a, which already exists. In this minimal example the argument does not affect the result; a real factory should use its input for a meaningful mapping where appropriate. It still cannot add enum instances. The distinction and permitted uses are set out in the diagnostic documentation.
For application code, choose the expression that matches the job:
- The desired value is already known: pass the enum constant itself, such as
Mode.compact. - A callback receives input and should map it to a declared value: provide a factory constructor and pass that factory reference if its signature fits the callback type.
- You need to construct arbitrary new objects from callback input: use an ordinary class constructor tear-off, not an enum generative constructor.
A factory does not change the enum’s closed set. It only provides a route to return a member of that set.
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Dart 3.13 primary constructors change syntax, not enum construction
The Dart language documentation reflects Dart 3.13.3, and primary constructors offer a shorter declaration style for enhanced enums. For example, the documented primary-constructor form can put a declaring parameter in the enum header:
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enum Color(int value) {
red(0),
green(1),
blue(2);
final int value;
}Primary constructors are implicitly constant for enums. This syntax changes how the declaration is written, not the fixed-instance rule or the restriction on tearing off a generative enum constructor. See Dart primary constructors and the enum documentation. Use this newer syntax only when the project’s Dart language version supports it; the traditional enhanced-enum form remains available from language version 2.17.
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