A value of type 'Null' can't be returned by the 'onError' handler

Question

I can't return a null from catchError handler of dart Future. I can do it using try catch but I need to use then catchError.

Using try catch

 Future<bool?> test() async {
    try {
      return await someFuture();
    } catch (e) {
      return null;
    }
  }

// Works without error

But when using then catchError

  Future<bool?> test() {
    return someFuture().catchError((e) {
      return null;
    });
  }

// Error: A value of type 'Null' can't be returned by the 'onError' handler because it must be assignable to 'FutureOr<bool>'

How do I return null if I encounter some error using then and catchError?

Solution

You should specify someFuture() signature, which probably returns Future<bool>

Future<bool> someFuture() async

Method catchError must return the same future type it is called on. You can overcome this by forwarding the value to then and convert it to Future<bool?>:

Future<bool?> test() {
    return someFuture()
 .then((value) => Future<bool?>.value(value))
 .catchError((e) {
    return null;
});

}

Comments

[julemand101]

The type of someFuture needs to be able to return null since catchError just gets the same return type as someFuture.

[Kedar Karki]

Making someFuture able to return null gives same error in runtime whereas previously it gave warning on compile time.