Dart - allow setting class property only once - class

Question

I'd like to have a property which is settable only once and then ignores further attempts to set it, so noone can accidentally set or reset the value.
I know that late can "kinda" do this, but when using late, neither the compiler, nor the IDE tell me that calling doOther() after calling doOnce() will cause an exception.
class Foo {
bool? _bar;
void doOnce() {
if (_bar == null) {
_bar = true;
print('done this once');
}
}
void doOther() {
_bar = false; // this should not be possible when doOnce() is done
print('done other');
}
}
void main() {
Foo foo = Foo();
foo.doOnce();
}

Answer 1

Using late final is as close as it gets:
class Foo {
late final int a;
Foo();
void doThing() {
a = 5;
}
void doAnotherThing() {
a = 3;
}
}
...
void main() {
final foo = Foo();
// Uncaught Error: LateInitializationError: Field 'a' has not been initialized.
// print(foo.a);
foo.doThing();
// Prints: 5
print(foo.a);
// Uncaught Error: LateInitializationError: Field 'a' has already been initialized.
// foo.doAnotherThing();
}
This gives you the runtime behavior you are looking for, but there is no solution that gives you static compile-time checking for assigning to a late (or late-ish) variable twice. That's because in order to know that your program is attempting to do so, the compiler would have to inspect every path of your program that could possibly assign to the late variable which could take a really long time.
In short, you are going to have to practice your own due diligence instead on relying on the compiler or static analyzer to inform you of your mistakes.